Ukraine’s President Petro Poroshenko has accused Russia of a “flagrant violation of international law” after Russian humanitarian aid lorries entered Ukraine without permission.
More than 100 Russian lorries had entered on Friday without any customs clearance or Red Cross escort, he said.
Reports say the first trucks have reached the rebel-held city of Luhansk in the eastern war zone.
Russia accused Ukraine of unreasonable obstruction and sent the convoy in, reports BBC.
Reports suggest the lorries are being escorted by rebel fighters.
The International Committee of the Red Cross (ICRC) said it was not part of the convoy “in any way”.
Russia’s foreign ministry has warned Ukraine not to take any action against the convoy, without specifying the consequences.
In a statement, President Poroshenko said “a column of more than 100 vehicles entered Ukrainian territory without a customs inspection, without border control or International Red Cross escort, which is a flagrant violation of international law”.
The risk of the Russian aid convoy causing a further deterioration in relations with Ukraine is now very high. Because the International Committee of the Red Cross is not part of the convoy, the government in Kiev may choose not to recognise it as a humanitarian mission.
The lorries are currently being accompanied by pro-Russian gunmen. So the convoy is likely to be stopped at the front line near the besieged city of Luhansk. Or if it tries to work its way around the Ukrainian government forces, there is a risk of it being attacked – accidentally or intentionally.
The Russian foreign ministry has already sent a clear warning that if the convoy is attacked, Moscow may take action.
Ukraine fears that the aid convoy of at least 260 lorries, which arrived at the border more than a week ago, is part of a broader Russian intervention in eastern Ukraine.
Russia denies accusations that it arms and trains the rebels in Luhansk and the neighbouring region of Donetsk, where four months of fighting have left more than 2,000 people dead and caused more than 330,000 people to flee their homes.
Russia’s aid convoy inside Ukraine, 22 August Russia’s aid convoy heads through Ukraine towards Luhansk
A Russian aid lorry enters Ukraine, passing a rebel sentry, 22 August Rebel fighters could be seen at the border as the lorries passed
A Ukrainian military helicopter over Kramatorsk, Donetsk region, 22 August The Ukrainian army continued operations in neighbouring Donetsk region on Friday
Luhansk has been without running water, power and phone communications for 20 days as government forces besiege it.
Citing “heavy shelling overnight” in Luhansk, an ICRC spokesperson in Moscow said it had concluded that it had not “received the necessary security guarantees from the fighting parties to allow us to escort the convoy at this time”.
Ukraine’s prime minister said the move was Russia’s attempt to distract the world from the fallout of down of the Malaysia Airlines flight MH17 last month.
It was shot down by Russian military specialists not by “drunken” separatists, Arseny Yatsenyuk said in briefing to journalists.
Ukraine’s foreign ministry called on “international partners” to unite in condemning Russia’s “illegal and aggressive actions”.
The European Union’s foreign affairs chief Catherine Ashton urged Russia to reverse its decision as her spokesman said the convoy was “a clear violation of the Ukrainian border”.
As the lorries passed over the border in a rebel-held sector near the Russian town of Kamensk-Shakhtinsky, reporters at the scene saw rebel fighters in front of the convoy.
According to the independent Organisation for Security and Co-operation (OSCE) observer mission, a total of 134 trucks, of which 34 have been inspected, had crossed over the border by 14:20 local time (11.20 GMT).
They were occupied by 12 “support vehicles”, including an ambulance.
The Russian foreign ministry warned against any attempts to sabotage the “purely humanitarian mission”, saying it had been prepared in an atmosphere of full transparency with Ukraine and the ICRC.
But delays in Ukrainian clearance for the convoy had “become unbearable”, it said in a statement (in Russian).
“All excuses for blocking the delivery of aid to people in the area where this humanitarian catastrophe is happening have been exhausted,” it added.
Foreign journalists were allowed to look into the Russian lorries earlier and found they contained humanitarian supplies such as baby food and cereals.
In a statement on its website (in Russian), Luhansk’s official council reported on Friday that the dire situation in the city remained unchanged with no halt in the bombardment.