Russia’s decision to send more than 100 aid lorries into war-torn eastern Ukraine without permission has been widely condemned in the West.
The EU and the US called for them to be withdrawn, saying their presence violated Ukraine’s sovereignty. Nato said the move would fuel the crisis.
But Russia said further delays were unacceptable – and also denied that its troops and artillery were in Ukraine.
The first lorries have now reached the besieged rebel-held city of Luhansk.
The UN has held an emergency meeting to discuss the convoy’s entry, which Ukraine has described as an “invasion”.
Meanwhile, Lithuania – a fierce critic of Russia’s actions in Ukraine – has said its honorary consul in Luhansk has been murdered “by terrorists”, a term routinely used by Ukraine for pro-Russian separatists.
In a phone call, US President Barack Obama and German Chancellor Angela Merkel cautioned that Russia was provoking a “dangerous escalation” in Ukraine and that the conflict there “has continued to deteriorate” since a Malaysian airliner was downed last month over rebel-held territory in the east with the loss of all 298 people on board.
The White House and the Ukrainian government have both described the deployment of the convoy as a flagrant violation of Ukraine’s sovereignty.
Mrs Merkel, who is travelling to Kiev on Saturday for talks, said she was “deeply concerned” by the developments.
Analysis: Jonathan Marcus, BBC defence and diplomatic correspondent
Nato’s condemnation of the entry of Russia’s aid convoy into Ukraine marks a significant hardening of its tone. The statement speaks of “a major escalation in Russian military involvement in eastern Ukraine since mid-August, including the use of Russian forces”.
Indeed Nato goes further, insisting that Russian artillery is being used against Ukrainian forces, with the fire coming both from across the border in Russia, and from within Ukraine itself. Western military sources speak of “substantial” Russian military deployments inside Ukraine with battalion-size elements on the ground.
All this, taken with what Nato describes as the transfer of “large quantities of advanced weapons” to separatist groups in eastern Ukraine, suggests a concerted effort by Moscow to prop up the defences of Luhansk and Donetsk. Russia has consistently denied such charges but Russian armour has been seen crossing into Ukraine and marshalled near the border.
In other reactions to the convoy:
- Nato chief Anders Fogh Rasmussen said Friday’s developments were “even more worrying as they coincide with a major escalation in Russian military involvement in eastern Ukraine since mid-August, including the use of Russian forces”
- Pentagon spokesman John Kirby said the convoy was a “pretence”, that allowed Russia to encroach on Ukraine through unauthorised means
- The UK’s UN ambassador, Sir Mark Lyall Grant, said it was a provocative act that had “nothing to do with humanitarianism”
- The International Committee of the Red Cross (ICRC) said it was not part of it “in any way”.
But speaking after the UN Security Council meeting, Russia’s UN ambassador Vitaly Churkin accused Western powers of distorting reality.
“Sometimes it reminded me of the kingdom of crooked mirrors because some members of the Council were not concerned about the fact hundreds of civilians are dying.”
Earlier he said that Russia had lost patience with Ukraine’s chain of command and had had to act to save perishable goods and he hoped the ICRC would help now distribute the aid.
“We waited long enough. And it was time to move, and this is what we did,” he said.
Mr Churkin accused the Ukrainian foreign ministry of issuing blatant untruths in relation to the convoy, arguing that Ukrainian officials were informed as far back as 12 August as to the humanitarian nature of its cargo.
He rebuffed Nato’s accusations of military support for the Ukrainian separatist forces, saying the body had no proof – adding there were no Russian troops or artillery on Ukrainian soil.
He also accused Lithuania’s UN delegation of “torpedoing all productive, constructive initiatives we’ve had in the Security Council”, in particular Russia’s proposal calling for a ceasefire.
Mr Churkin’s Ukrainian counterpart, Oleksandr Pavlichenko, said he was deeply concerned about the convoy. He said that border guards had begun inspecting some of the lorries on Thursday afternoon, but the Russians had decided to go ahead without waiting for them all to be cleared.
Foreign journalists were allowed to look into the Russian lorries earlier this week, and found they contained humanitarian supplies such as baby food and cereals.
Reporters at the scene saw rebel fighters in front of the convoy as it crossed the border near the town of Kamensk-Shakhtinsky.
Four months of fighting in eastern Ukraine have left more than 2,000 people dead and caused more than 330,000 people to flee their homes.