Obama condemns Russian 'aggression'

US President Barack Obama has condemned what he called Russian "aggression" in Ukraine as he welcomed counterpart Petro Poroshenko to the White House for the first time.

He also praised the Ukrainian leader on Thursday for his "difficult" move to offer limited self-rule to parts of his war-torn country's separatist east as part of a peace plan reached with Moscow.

Obama, seated side-by-side with Poroshenko in the Oval Office, in what officials said was a deliberate show of support aimed at Moscow, praised his visitor's leadership - which he said had been "critical, at a very, very important time in Ukraine's history".

"Unfortunately, what we have also seen is Russian aggression, first in Crimea and most recently in portions of eastern Ukraine."

Obama said Russia's moves had violated Ukraine's sovereignty and were also designed to undermine Poroshenko's reform efforts.

The US president praised Poroshenko for pushing legislation through Ukraine's parliament offering self-rule to some eastern Ukrainian districts in a bid to appease separatists.

He said the legislation, which was "very difficult" for Poroshenko, should give eastern Ukrainians confidence that their rights were fully respected.

"Those were not easy laws that President Poroshenko passed," Obama said.

"But I think they indicated his commitment to an inclusive Ukraine."

Poroshenko said he wanted a ceasefire between his forces and rebels to hold and to solidify into a lasting peace, and thanked Obama for his support.

"The only way we can de-escalate the situation in the eastern part of our country is the peace process," he said.

He called for the withdrawal of Russian troops from Ukrainian territory, the closure of the Russian border to other troops and ammunition, and the release of Ukrainian "hostages".

"We really hope that the ceasefire, which already lasted for 12 days, will transform into a real peace," he said.