One of the top rebel leaders in eastern Ukraine claims that his forces have recently received 1,200 fighters who had undergone training in Russia. The claim came during a speech to leading rebels, apparently recorded on Friday and posted to YouTube by a pro-separatist media outlet. His language suggested the men had already crossed the border.

Alexander Zakharchenko, prime minister of the self-declared Donetsk People's Republic, also spoke about 150 armoured vehicles, of which 30 are tanks and the rest wheeled or tracked armoured personnel carriers. He said the men and equipment had gathered in an area near a "corridor" to the Russian border, though he did not specify whether the vehicles themselves had crossed from Russia.

"There are 1,200 individuals who have gone through training over a four-month period on the territory of the Russian Federation and who have been introduced here at the most decisive moment," said Zakharchenko. Many among the rebel fighters admit to being Russian volunteers or veterans who received training in the Rostov region before crossing the border.

It is unclear how much direct coordination by official Russian authorities has taken place, and there was no way to verify Zakharchenko's claims. However, it is significant that the claims came from the rebel side and not from Kiev.

The claims come after a week when Russia's actions on the border with Ukraine have been under renewed scrutiny. Journalists for the Guardian and Daily Telegraph observed a convoy of armoured vehicles accompanied by trucks with Russian military plates crossing over the border on Thursday night. Ukraine said it had destroyed part of a Russian column later that night, but did not provide any evidence. Russia denied any incursion had taken place.

Late on Friday, German chancellor Angela Merkel called Russian President Vladimir Putin and urged him "to help de-escalate the situation and in particular to halt the stream of weapons, military advisers and armed personnel into Ukraine," her spokesperson said.

In recent days several columns of Russian military hardware have been observed on the move close to the border. On Friday, a column of several dozen armoured personnel carriers and several dozen support trucks were spotted moving towards the border, many of them flying Russian flags and marked with "peacekeeping troops" insignia.

Despite the increased troop movement there are some signs of negotiations behind closed doors. The heads of Russia and Ukraine's presidential administrations met in the Russian city of Sochi on Friday and agreed that the countries' foreign ministers would meet on Sunday in Berlin.

Several separatist leaders have left their posts, including Valery Bolotov, head of the self-proclaimed Luhansk People's Republic, and Igor Strelkov, defence minister for the Donetsk rebels. Strelkov, a Russian citizen who says he was an agent of Russia's FSB security service until last year, has been one of the key figures in the rebel movement. Other rebels said he was "resting" before being given a new task. It is unclear if he has returned to Russia. Ukrainian forces have made major gains against the rebels in recent weeks, and are closing in on the main cities of Donetsk and Luhansk, though their advance has come with heavy civilian casualties.

Russia has also let 58 Ukrainian border guards onto its territory, with the expectation that they will be allowed to inspect the humanitarian convoy currently stationed close to the town of Kamensk-Shakhtinsky around 20 miles from the border with Ukraine.

The convoy, which set out from a military base near Moscow on Tuesday, has caused controversy in Kiev, where some officials believed it could be cover for military intervention. Russia has insisted it only contains food and aid, and showed some of the contents to journalists on Thursday and Friday. Negotiations are under way to allow the trucks to cross the border with accompaniment from the International Red Cross.