A controversial Russian aid convoy has crossed the border into Ukraine, in a move described by Kiev as a "direct invasion". The first trucks in the convoy arrived in the war-torn city of Luhansk late on Friday afternoon.

After Russia's foreign ministry announced that it could not wait any longer on the convoy – which had been stuck at the border for more than a week – the vehicles passed through a Ukrainian border post controlled by pro-Russia fighters. Rebels in cars escorted the convoy, which moved ahead without observers from the International Committee of the Red Cross (ICRC).

Reporters and officials at the border said all of about 260 trucks in the convoy had crossed the border by Friday afternoon. Russian outlet LifeNews showed footage of men unloading bags from the trucks in Luhansk, where it said 100 trucks had arrived. It also reported some mortar fire had landed near the convoy in the city although other reports said the convoy had not been targeted.

Some trucks were said to be leaving Ukraine and returning to Russia early on Saturday.

The head of Ukraine's security service, Valentin Nalyvaichenko, described the crossing of the border as a direct invasion but ruled out the use of force against the convoy. Nalyvaichenko argued that the convoy's drivers were Russian military forces members trained to drive combat vehicles and the half-empty trucks would be used to move weapons and bring the bodies of Russian fighters out of Ukraine.

Russia's move is sure to complicate peace talks between the Ukrainian president, Petro Poroshenko, and the Russian president, Vladimir Putin, set to take place on Tuesday in Minsk.

Western leaders fear the convoy could serve as a pretext for direct Russian intervention in the conflict between government forces and pro-Russia rebels which has been raging for the past four months in the Donetsk and Luhansk regions, suspicions Moscow has dismissed.

In a phone call with German chancellor Angela Merkel on Friday, Putin said in the light of Kiev's "obvious stalling", Russia had decided to send the convoy in because further delays would have been unacceptable, according to a Kremlin statement.

The move drew swift condemnation from the US and European countries. The White House warned of "additional consequences" if Russia does not remove the convoy.

"We very much condemn this flagrant violation of Ukrainian sovereignty which we saw today with the movement of this Russian convoy into Ukraine," said deputy national security adviser Ben Rhodes in a media briefing from the president's vacation home on Martha's Vineyard.

"We will be in touch today with our partners on the United Nations security council to discuss next steps. Russia should take the opportunity to remove this convoy from within Ukraine. If they don't, they will face additional costs and consequences from the United States and our partners in the international community".

In a separate statement issued on behalf of the White House national security council, the administration added: "In violation of its previous commitments and international law, Russian military vehicles painted to look like civilian trucks forced their way into Ukraine."

The US also accused Russian military forces of repeatedly firing into Ukrainian territory and warned that the convoy could be a "pretext for further Russian escalation of the conflict".

"Russia's decision today to send in its vehicles and personnel without the ICRC and without the express permission of the Ukrainian authorities only amplifies international concerns about Russia's true intentions," said the NSC statement. "It is important to remember that Russia is purporting to alleviate a humanitarian situation which Russia itself created – a situation that has caused the deaths of thousands, including 300 innocent passengers of flight MH17."

Nato secretary general Anders Fogh Rasmussen said: "The disregard of international humanitarian principles raises further questions about whether the true purpose of the aid convoy is to support civilians or to resupply armed separatists."

While 34 trucks were checked by Ukrainian and Russian officials on Thursday, the rest of the convoy had reportedly not been inspected by the Ukrainian side. Ukraine's military said 90 trucks that had not been checked by either side had been moving across the border on Friday.

In a combative statement, Russia's foreign ministry accused Kiev of "deliberately dragging out the delivery of the humanitarian aid" so it could complete a "military cleansing of Luhansk and Donetsk" by Independence Day celebrations on Sunday and before Poroshenko and Putin meet.

"It's impossible to suffer such an outrage, open lies and inability to negotiate any longer … Our column with humanitarian aid is starting to move toward Luhansk," the statement said.

"We warn against any attempts to disrupt this strictly humanitarian mission, which was prepared some time ago amid complete transparency and cooperation with the Ukrainian side and the ICRC," the statement added. "The responsibility for the possible consequences of provocations against the humanitarian convoy lie entirely on those who are ready to continue sacrificing human lives for their own ambitions and geopolitical plans, rudely trampling the norms and principles of international humanitarian law."

Ukraine's national security council said it had proposed negotiations between Ukraine and Russia's general staffs, but the Russian side had turned the offer down. The national security council spokesman, Andriy Lysenko, said the "responsibility for the safe movement of the column through territory in the Donbass [the historical name for Donetsk and Luhansk] not controlled by the Ukrainian authorities lies on Russia".

Street fighting and shelling in Luhansk have left tens of thousands of civilians without water, electricity or communications for more than two weeks. Representatives of the ICRC, who Kiev and Moscow had agreed would oversee the aid delivery to the city, were not accompanying the convoy on Friday. The convoy had reportedly been held up because the ICRC was waiting for safety guarantees from both sides in the conflict.

ICRC spokeswoman Anastasia Isyuk told the Guardian that 34 trucks had been checked by Russian and Ukrainian officials on Thursday but ICRC representatives did not accompany the convoy on Friday.

"Because of the volatile security situation, with heavy shelling continuing through the night in Luhansk, we do not believe we have received sufficient security guarantees from the fighting parties to allow us to escort the convoy at this time," Isyuk said. "The convoy of Russian aid is now moving. However, we are not part of that convoy in any way."

But the Russian Red Cross was "ready to take part in escorting the convoy" and was waiting for a response from the ICRC, its director Raisa Lukuttsova told Interfax news agency.

The Ukrainian border service said in a statement that its group of customs and border patrol officials had been "barricaded in the Russian border crossing at Donetsk". Lysenko told reporters on Friday that Kiev was waiting for information from the foreign ministry and Red Cross before deciding whether to stop the convoy from moving further.

Dmitry Tymchuk, a defence analyst with close links to the Kiev government, said Moscow was "openly continuing its provocation under the guise of 'humanitarian aid for the residents of Donbass".

Government forces have claimed tactical victories in fighting around these two rebel strongholds in recent weeks but have yet to capture them. Rebels shot down a Ukrainian Mi-24 helicopter in the Luhansk region on Wednesday but Kiev did not immediately release this information so as not to disrupt the search for it, Lysenko said on Friday.