One of Vladimir Putin’s senior advisors branded Petro Poroshenko a “Nazi” ahead of the signing of a controversial EU deal, straining the relationship between Russia and Ukraine even further.

Sergei Glazyev’s comments came shortly before the Ukrainian president signed the full association agreement at the EU summit in Brussels amid a precarious ceasefire between government troops and pro-Russian rebels.

Mr Glazyev, Mr Putin's presidential adviser on regional economic integration, also described Mr Poroshenko’s endorsement of the deal as “illegitimate” and accused Europe of attempting to push Ukraine to sign the agreement, which is expected to take place later today “by force”.

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He told the BBC: "They organised [a] military coup in Ukraine, they helped Nazis to come to power. This Nazi government is bombing the largest region in Ukraine."

When asked if he believed Mr Poroshenko was a Nazi, he replied "of course", adding: "I think after the signing of the agreement with EU, [the] European public will be... surprised when this Nazi Frankenstein, which was born by the Euro bureaucrats and some politicians, will knock on the European countries' doors."

His inflammatory remarks follow news that four members of the Vienna-based Organisation for the Security and Co-operation in Europe (OSCE) had been released in what Alexander Borodai, head of the Donetsk People's Republic, claimed was part of a goodwill gesture.

Shape Created with Sketch. Ukraine crisis aftermath: Slavyansk residents show the scale of damage caused by weeks of unrest Show all 10 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. Ukraine crisis aftermath: Slavyansk residents show the scale of damage caused by weeks of unrest 1/10 Ukraine crisis Yekaterina Len (61) cries outside her ruined house in Slovyansk 2/10 Ukraine crisis Yekaterina Len (61) cries inside her ruined house following a shelling as her grandson stands behind her in Slovyansk 3/10 Ukraine crisis Neighbors of Yekaterina Len react at Len's destroyed house following a shelling as they speak to journalists in Slovyansk 4/10 Ukraine crisis Yekaterina Len (61) and her granddaughter comfort each other at their destroyed house following a shelling in Slovyansk 5/10 Ukraine crisis Neighbors of Yekaterina Len react at Len's destroyed house following a shelling in Slovyansk 6/10 Ukraine crisis Relatives of Yekaterina Len try to clean debris in her destroyed house following a shelling in Slovyansk 7/10 Ukraine crisis Neighbors comfort Yekaterina Len in her destroyed house following a shelling in Slovyansk 8/10 Ukraine crisis Local residents look at flame from a damaged gas pipe after an impact of a mortar bomb, during fighting between Ukrainian government troops against pro-Russian militants in Slovyansk 9/10 Ukraine crisis Local residents talk to each other as they look at flame from a damaged gas pipe after an impact of a mortar bomb in Slovyansk 10/10 Ukraine crisis Lawmakers and officials from eastern Ukraine poured criticism on the fledging central government, accusing it of ignoring legitimate grievances of the regions which have been overrun by pro-Russia militia fighting for independence 1/10 Ukraine crisis Yekaterina Len (61) cries outside her ruined house in Slovyansk 2/10 Ukraine crisis Yekaterina Len (61) cries inside her ruined house following a shelling as her grandson stands behind her in Slovyansk 3/10 Ukraine crisis Neighbors of Yekaterina Len react at Len's destroyed house following a shelling as they speak to journalists in Slovyansk 4/10 Ukraine crisis Yekaterina Len (61) and her granddaughter comfort each other at their destroyed house following a shelling in Slovyansk 5/10 Ukraine crisis Neighbors of Yekaterina Len react at Len's destroyed house following a shelling in Slovyansk 6/10 Ukraine crisis Relatives of Yekaterina Len try to clean debris in her destroyed house following a shelling in Slovyansk 7/10 Ukraine crisis Neighbors comfort Yekaterina Len in her destroyed house following a shelling in Slovyansk 8/10 Ukraine crisis Local residents look at flame from a damaged gas pipe after an impact of a mortar bomb, during fighting between Ukrainian government troops against pro-Russian militants in Slovyansk 9/10 Ukraine crisis Local residents talk to each other as they look at flame from a damaged gas pipe after an impact of a mortar bomb in Slovyansk 10/10 Ukraine crisis Lawmakers and officials from eastern Ukraine poured criticism on the fledging central government, accusing it of ignoring legitimate grievances of the regions which have been overrun by pro-Russia militia fighting for independence

"We don't expect anything in return - we freed them without any pre-conditions," he said.

The OSCE said four other members were also kidnapped by pro-Russian separatists in eastern Ukraine a month ago. Spokesman Steffen Seibert said leaders of Russia, Ukraine, Germany and France had agreed on Wednesday in a telephone call that the hostages should be freed without delay.

Thousands of Ukrainians are reported to have lined up at the border with cars stuffed with their belongings to cross into Russia as the shaky cease fire entered its final hours today.

Russia claims tens of thousands of Ukrainians have come in the two-and-a-half months since Ukraine's government began fighting separatists in the east, a heavily industrial region with a large population of ethnic Russians, many of whom feel strong ties to Moscow.

Air strikes and artillery attacks by the Ukrainian military have infuriated many residents. Many crossing the border said they were fleeing the fighting, which has killed more than 400 people since mid-April by the United Nations' estimate.

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Talks to possibly extend the truce in Donetsk and Luhansk are set to take place on Friday. It will be the second round of talks since Monday in which the rebel leaders have participated.

Mr Poroshenko has urged Russia to support his peace plan "with deeds, not words" a call echoed by the US Secretary of State John Kerry ahead of the summit of EU leaders, who will be considering a new round of sanctions against Russia.