Several dozen trucks, from a convoy of about 270, crossed the border around noon. Soldiers carrying automatic rifles and wearing camouflage, some bearing the markings of the rebels in eastern Ukraine, cleared the road to let the convoy move past.
There were no signs of Russian military vehicles or any other indications of an armed escort by Russian troops. The United States and its European allies have warned that any crossing of the border by Russian military vehicles, even under the pretext of protecting the aid convoy, would be regarded as an invasion.
The main highway from the border crossing at Izvaryne to Luhansk has seen heavy fighting over the past week, as Ukrainian forces pressed their military campaign against the separatists. The rebel forces have been driven out of a string of towns and villages but are still holding out in Luhansk and Donetsk.
Under the arrangements agreed to by the two countries, officials from the International Committee of the Red Cross were to escort the trucks to Luhansk. They decided not to proceed after heavy shelling around the city during the night, Ewan Watson, a spokesman for the Red Cross, said in Geneva.
He added that Ukrainian forces would allow the convoy to reach Luhansk, because “it is easy to shoot but the consequences would be very destructive.” He said Ukraine would adopt a different approach if it turned out that, after reaching Luhansk, the convoy “has other equipment, not just humanitarian aid.”
“This is a provocation,” said Andriy Lysenko, Ukraine’s military spokesman. “They expect us to attack the convoy.”
Ukraine has from the start viewed Russia’s aid convoy, which left Moscow on Aug. 12, with deep suspicion, worrying that the vehicles could be carrying weapons or be part of a ruse by Moscow to support the pro-Russian separatists, or possibly an attempt to provoke Ukraine into an ill-advised attack.
Spreading the conspicuously large white aid trucks through Luhansk could effectively impose a cease-fire, essentially daring the Ukrainians to fire at vehicles that have been sent to provide desperately needed humanitarian assistance. Any respite in Ukraine’s military offensive could allow rebels to dig in further, and indefinitely postpone any attempt to oust them.
The decision to send in the aid trucks in defiance of Kiev suggested an attempt by Mr. Putin to calm nationalist complaints that he has not done enough to prevent a rebel defeat and marked the latest in a long series of surprise moves by the Russian president, a judo practitioner, to put Ukraine and the West off balance.
While denying that it supports the rebels, despite a steady flow of arms and fighters into eastern Ukraine from Russia, the Kremlin has tied itself to their fate by whipping up a nationalist fervor with vows to protect Russians beyond Russia’s borders.
Luhansk has come under heavy military pressure in recent days from Ukrainian forces. The rebel city’s recapture by Ukraine would deliver a humiliating blow to Mr. Putin, who has faced mounting calls from hard-line nationalists in Russia to intervene decisively to stave off defeat for the Russian-speaking and often ethnically Russian separatists.
The trucks are traveling toward Luhansk, a war-ravaged rebel-held city where the bullet-riddled body of the acting honorary consul for Lithuania, a 39-year-old Ukrainian national named Mykola Zelenets, was found on Thursday. He was kidnapped earlier this month by armed separatists, the Lithuanian ambassador to Ukraine, Petras Vaitiekunas, said.
Rather than an invasion, however, the arrival of Russian trucks — only 34 of which Ukrainian officials inspected Thursday evening on the Russian side of the border, and found to contain buckwheat, rice, sugar and water — appeared to be a Russian effort to stall an accelerating offensive by Ukrainian forces against beleaguered pro-Russian rebels in eastern Ukraine. The Ukrainian government has suggested it would be reluctant to attack with a convoy interposed.
“They will not add anything to this escalation,” he said, adding that Russia’s decision to move in its aid trucks, many of which appeared half empty to Western journalists allowed to see them earlier this week, only strengthened suspicions that the Kremlin’s humanitarian effort “is a smoke screen for something else.”
In a telephone interview after a meeting with Mr. Poroshenko, the Lithuanian foreign minister, Linas A. Linkevicius, described the Russian move as “a serious escalation” but said Ukraine’s president had made clear there would be “no attack” on the trucks by Ukrainian forces.
“It is important to remember that Russia is purporting to alleviate a humanitarian situation which Russia itself created,” Caitlin Hayden, the spokeswoman for the National Security Council, said in a statement. “If Russia really wants to ease the humanitarian situation in eastern Ukraine, it could do so today by halting its supply of weapons, equipment, and fighters to its proxies,” she added.
That brought a curt response from a spokeswoman from the Obama administration.
“All the excuses to delay the delivery of aid to people in the area of a humanitarian catastrophe are exhausted,” the ministry said. “The Russian side has made a decision to act. Our column with humanitarian cargo starts moving toward Luhansk.”
The Russian Ministry of Foreign Affairs issued a long statement saying in essence that it had authorized the crossing because it was fed up with stalling by the government in Kiev. Russian news agencies quoted a spokesman for Mr. Putin as saying that he had been informed of the convoy’s movements.
The comments by Mr. Poroshenko suggested that Ukraine would limit its response to verbal protests and not use force against the Russian vehicles, although it was unclear whether volunteer Ukrainian battalions of sometimes shaky discipline would respect calls for restraint.
But Ukraine stepped back from earlier threats to use “all forces available” to halt any Russian vehicles that crossed the frontier without its full accord, and Mr. Poroshenko told the visiting foreign minister of Lithuania, “We will do our best to ensure that this does not lead to more serious consequences.”
In a news briefing, the Pentagon spokesman, Rear Adm. John Kirby, condemned the convoy as an “unauthorized entry into Ukraine” and called for the vehicles’ immediate withdrawal. The move into Ukraine, without the Red Cross escorts that had been agreed upon, drew angry accusations from Ukraine that Moscow had broken its word and mounted what Valentyn Nalyvaichenko, the head of Ukraine’s Security Service, called a “direct invasion.”
Ms. Merkel spoke by telephone with the Russian and Ukrainian presidents on Friday and “expressed her great concern” over Russia’s unilateral decision to move its aid trucks into Ukraine, her spokesman said. She also spoke with President Obama and, according to the White House, both leaders agreed that the arrival of the convoy represented “a violation of Ukraine’s sovereignty and territorial integrity.”
But the NATO statements added new pressure on Moscow before a flurry of diplomacy in coming days, including a visit to Kiev on Saturday by the German chancellor, Angela Merkel, and a scheduled meeting next week between President Petro O. Poroshenko of Ukraine and his Russian counterpart, Mr. Putin, in Minsk, Belarus.
A NATO spokeswoman, Oana Lungescu, also said that the alliance had receive multiple reports of the direct involvement of Russian airborne, air defense and special operations forces in Eastern Ukraine.
The NATO allegations are based on intelligence reports from several alliance members, Western officials said, and the allegation generally echoed Ukrainian claims in recent days of an expanding Russian military involvement in support of pro-Russian rebels who are battling to hold off a Ukrainian offensive.
Mr. Rasmussen did say how many Russian artillery pieces had been moved into Ukraine or where they were located, but one Western official said the number of Russian-operated artillery units was “substantial.”
Russia’s Permanent Mission to NATO, in a statement, accused the alliance of indifference to humanitarian suffering in eastern Ukraine and described its protests over the entry of a Russian aid convoy into Ukraine without Red Cross escorts as “another cynical attempt to cover the crimes of Ukrainian authorities.”
There has been a “major escalation in Russian military involvement in eastern Ukraine since mid-August, including the use of Russian forces,” Secretary General Anders Fogh Rasmussen of NATO said in a statement. “Russian artillery support — both cross-border and from within Ukraine — is being employed against the Ukrainian armed forces,” Mr. Rasmussen added.
NATO officials said that the Russian military had moved artillery units inside Ukrainian territory in recent days and was using them to fire at Ukrainian forces. Russia has repeatedly denied sending troops or military hardware into Ukraine, just as it denied any link to the unidentified gunmen who paved the way for Moscow’s annexation of Crimea — until President Vladimir V. Putin stated in April that Russian troops were “of course” involved.
KIEV, Ukraine — Russia on Friday escalated tensions with Ukraine to the highest level since its stealthy invasion of Crimea in the spring, sending more than 200 trucks from a long-stalled aid convoy into rebel-held eastern Ukraine over the objections of Kiev and, NATO said, conducting military operations on Ukrainian territory.