Sources in Moscow have admitted that a number of men captured inside Ukraine were indeed serving Russian soldiers, but said they crossed the border by mistake. The admission comes as President Vladimir Putin meets his Ukrainian counterpart, Petro Poroshenko, for talks in Minsk. The pair shook hands ahead of the talks on Tuesday afternoon.

Videos were released by Ukrainian authorities of interrogations of the prisoners, who said they were serving Russian army officers. One said he had not been told exactly where they were going, but had an idea he was inside Ukraine. There was no immediate confirmation of the authenticity of the recordings, but the fact that Russian wire agencies ran a defence ministry admission that soldiers had indeed crossed into Ukraine suggested that the footage was genuine.

"The soldiers really did participate in a patrol of a section of the Russian-Ukrainian border, crossed it by accident on an unmarked section, and as far as we understand showed no resistance to the armed forces of Ukraine when they were detained," a source in Russia's defence ministry told the RIA Novosti agency.

Ukraine said it had captured 10 Russian soldiers, though it did not state how they were caught. Weapons and fighters are able to cross the porous border freely, but until now there has never been confirmation that serving Russian soldiers were active inside Ukraine, despite repeated claims from Kiev and some circumstantial evidence.

This makes the videos released on Tuesday all the more significant if authenticated.

Two weeks ago, the Guardian saw a convoy of armoured personnel carriers and support trucks with Russian military plates cross an unmarked section of the border near the town of Donetsk.

Russia furiously denied that any incursion had taken place, and said the column was on a "border patrol" mission that stayed strictly on the Russian side.

The incursions by Russian soldiers are likely to be discussed at the meeting between Putin and Poroshenko on Tuesday.

The official reason for the summit is a meeting of the nations of the Customs Union, which includes Russia, Belarus and Kazakhstan, but all eyes will be on the meeting between Putin and Poroshenko, only their second since the Ukrainian president won elections in May.

"As well as trade and energy issues, the priority for the meeting in Minsk is the question of peace," said Poroshenko during a televised discussion with Belarusian president Alexander Lukashenko before the wider talks began.

Russia has called for an immediate ceasefire in eastern Ukraine, but Kiev wants to finish its "anti-terrorist operation" to win back control of the whole country. Both leaders are under pressure from domestic audiences not to make concessions, and there is little hope of a major breakthrough.