Ukraine declared on Friday that Russia had launched a "direct invasion" of its territory after Moscow sent a convoy of aid trucks across the border into eastern Ukraine where pro-Russian rebels are fighting government forces.

Moscow, which has thousands of troops on the Russian side of the border, warned against any attempt to "disrupt" the convoy but did not specify what action it was prepared to take if Kiev's forces intervened.

Kiev, for its part, said Ukrainian forces would not attack the convoy and had allowed it to pass to avoid “provocations.”

The Ukraine conflict has driven relations between Moscow and the West to their lowest level since the Cold War, with Western states imposing sanctions on Moscow and the Kremlin retaliating. NATO has deployed extra troops in member states bordering Russia.

About 260 trucks had been waiting at the border for permission for over a week. A Reuters witness said the trucks that crossed the border were now heading toward the rebel stronghold of Luhansk escorted by a small number of pro-Moscow separatist fighters.

"They passed into Ukraine without clearance or participation of the International Red Cross or [Ukrainian] border guards," Ukrainian military spokesman Andriy Lysenko told journalists.

"We consider this a direct invasion by Russia of Ukraine," Ukrainian state security chief Nalivaychenko said in his statement to journalists.

In response to a question whether Ukraine would use airstrikes against the convoy, Nalivaychenko said: "Against them, no."

But Ukrainian authorities said the convoy would pass through an area where the rebels were firing and that therefore its security could not be guaranteed.

NATO Secretary General Anders Fogh Rasmussen condemned Russia on Friday for sending the "so-called humanitarian convoy" into Ukraine without the consent of Kiev and accused it of escalating the crisis.

"It can only deepen the crisis in the region, which Russia itself has created and has continued to fuel," Rasmussen said in a statement.

Ukrainian President Petro Poroshenko said more than 100 trucks had crossed the border, of which only some had been checked earlier by Ukrainian officials inside Russian territory.

Rasmussen said that NATO had also seen transfers of large quantities of advanced weapons, including tanks, armored personnel carriers and artillery to separatist groups in eastern Ukraine.

"NATO is observing an alarming build-up of Russian ground and air forces in the vicinity of Ukraine," Rasmussen said.

The Luhansk region has been a major focus of conflict in recent days between rebels, who have declared an independent republic, and Ukrainian forces. Luhansk city itself has seen fighting.

Moscow had earlier expressed impatience with holdups at the frontier.

"All excuses to delay sending aid have been exhausted," the Russian foreign ministry said in a statement. "The Russian side has taken the decision to act.

"We warn against any attempts to disrupt this purely humanitarian mission," it added.

"Responsibility for any possible consequences of provocations ... will lie, completely and entirely, with those who are prepared to further sacrifice human lives for the sake of their ambitions and geopolitical ploys."

The ICRC, which both Moscow and Kiev had agreed should supervise the convoy, said it was not escorting it "due to the volatile security situation.”

Kiev has been using troops, artillery and air power in an attempt to quell a separatist rebellion that broke out soon after Russia annexed the Crimean peninsula from Ukraine in March. The last few weeks has seen a string of rebel defeats in a conflict that has killed over 2,000 people.

Kiev and Western capitals have expressed concern that the convoy could be used as a pretext for some form of direct Russian military intervention. Russia, at odds with Kiev since popular protests drove a pro-Russian president from office, denies the accusation.

Russia says the aid trucks contain food, medical supplies, water and some clothing.

Ukrainian President Petro Poroshenko said on Thursday he would call on President Vladimir Putin to rein in pro-Russian separatists when the two men meet next week and told the Kremlin chief he had "a strong country, a strong army" behind him.

German Chancellor Angela Merkel is scheduled to visit Kiev on Saturday to show her support for Poroshenko — but diplomats say she is also bearing a message that he should consider calling a cease-fire so as not to incur a backlash from Putin.

The dispatch of the Russian convoy onto Ukrainian territory complicates the situation, presenting Ukraine with a choice of whether or how to confront what it sees as an illegal incursion.

After four months of fighting in the industrial, Russian-speaking eastern Ukraine, the area faces a humanitarian crisis, lacking supplies of food, medicine and clean water.

Al Jazeera and Reuters