The West had demanded that Russia withdraw its disputed trucks after the Kremlin unilaterally sent them to the insurgent stronghold of Lugansk yesterday 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.
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.
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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.