A man walks near a burnt shop, which according to locals was recently destroyed by shelling, at a market in Donetsk, eastern Ukraine, January 15, 2015. [Photo/Agencies]

WASHINGTON -- US President Barack Obama and German Chancellor Angela Merkel on Thursday voiced concern over an upsurge in violence in eastern Ukraine, calling for a "full and prompt" implementation of ceasefire deals.

In their phone conversation, the two leaders "expressed concern " about the increase in violence in eastern Ukraine, where more than 4,800 people have been killed and over 1 million others displaced in fighting between government troops and armed separatists that broke out in April last year.

Twelve passengers were killed on Tuesday when a bus came under heavy shelling at a checkpoint fortified by government forces near the town of Volnovakha in Donetsk region.

Obama and Merkel reiterated a call for "full and prompt" implementation of the Minsk agreements in order to reach a " lasting and peaceful" resolution to the conflict, the White House said in a statement.

The agreements, reached in September last year in the Belarussian capital of Minsk between representatives from Kiev and the rebel leadership, have never been fully implemented.

The foreign ministers of Russia, Ukraine, Germany and France met in Berlin on Monday over how to defuse the crisis in Ukraine's east, but a follow-up summit between the leaders, originally slated for Thursday in Astana, the capital of Kazakhstan, had been put off due to differences between the conflicting parties.

In their phone talks, Obama and Merkel "discussed their support for a robust package of international financing for Ukraine as it implements an ambitious series of reforms," the White House said.

The United States on Tuesday promised an additional $2 billion in loan guarantees to match the economic and political reforms pursued by Ukraine, while the European Union made a similar pledge of 1.8 billion euros ($2.1 billion) in additional aid a week ago to help prevent Kiev from bankruptcy.