Kiev (AFP) - German Chancellor Angela Merkel arrived in Kiev Saturday for crisis talks with Ukraine's pro-Western leaders, as a controversial aid convoy from Moscow began crossing back from the war-torn east of the country to Russia.

The West had demanded that Russia withdraw its disputed trucks after the Kremlin unilaterally sent them to the insurgent stronghold of Lugansk on Friday in a move Kiev decried as an "invasion".

An observer for the Organisation for Security and Cooperation in Europe (OSCE) at the border told AFP that some of the white lorries had returned to Russia but could not specify how many were still inside Ukraine.

US President Barack Obama and Merkel had warned following a telephone call that the Russian decision to send in the convoy marked a "dangerous escalation" of the four months of conflict in Ukraine with Western fears mounting that it could presage Moscow sending in troops.

The European Union and the United States both called for Moscow to pull out the trucks immediately or face further isolation as they drove cross-border tensions to a new high ahead of an already tricky visit to Kiev for the German leader.

Merkel will have to tread a fine line in Ukraine, showing firm support for Kiev's pro-Western leaders while also pushing for them to show restraint in their increasingly successful -- but brutal -- offensive.

Merkel, the most influential Western leader to visit Ukraine's pro-Western government, will hold talks with President Petro Poroshenko, three days ahead of the first meeting in months between Poroshenko and Russian leader Vladimir Putin in Minsk alongside top EU officials.

Poroshenko has pledged to "talk peace" with Putin but insists that an end to the conflict that has cost more than 2,200 lives can be achieved only if pro-Kremlin fighters are pulled out of Ukrainian territory.

Germany's vice chancellor, Sigmar Gabriel, said in a newspaper interview that establishing a federal Ukraine would be the only "viable path" to ending the crisis, a statement likely to set off alarm bells among Kiev's leadership who have fiercely opposed such plans.

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Analysts said that the visit by Merkel would need "to consolidate the position of Kiev and the EU" in the run-up to the meeting with the Russian strongman or risk Moscow exploiting differences between the two sides.

"Putin will play on any contradictions between Kiev and Brussels and on Germany's objective economic interests," Oleksandr Sushko, the director of the Kiev-based Institute for Euro-Atlantic Cooperation, told AFP.

The European Union and the United States have already slapped the toughest sanctions on Moscow since the Cold War over the crisis in Ukraine, a move that led Russia to ban agricultural products from much of the West.

- Catch the convoy -

Meanwhile the fate of the controversial Russian aid trucks remains the most pressing issue.

Russian news agencies reported that dozens of lorries had crossed back over the border after they were seen unloading their cargoes in the stricken rebel bastion of Lugansk, which has been without power or water for weeks.

There was no way to verify whether all of the roughly 250 trucks were pulling out or what exactly they transported into Ukraine as Kiev's officials managed to check only a handful of them.

An official from Lugansk's city council told AFP on condition of anonymity that several dozen trucks were unloaded and that rebels had begun organising for aid to be handed out to local residents.

The official could not say how many trucks remained in the city.

Kiev and the West fear that the convoy could be used to bolster the struggling rebels or provide Moscow with a pretext to stage a full-scale intervention.

- Fresh casualties -

Fighting has intensified ahead of the fresh round of diplomacy as Ukraine has pressed on with its punishing push into dwindling rebel-held territory in a bid to deal a knockout blow to the ailing insurgents.

Kiev has accused Russia of stepping up arms supplies to the flagging rebellion but Moscow has denied these claims and demanded that Ukrainian forces immediately cease fire.

In a reminder of the bloody toll the conflict is taking on ordinary people, AFP saw the bodies of two civilians killed by shelling sprawled on the pavements of the main rebel stronghold of Donetsk on Saturday.

Donetsk, which had a population of some one million before the conflict, has been pummelled by artillery bombardments as Ukraine forces have tightened the vice on rebel hunkered down there.

Insurgent-held areas are facing a humanitarian crisis with water cut off and food supplies dwindling.

More than 400,000 people are estimated to have fled the conflict in the east since April.