Donald Trump heralded 'impressive' prospects for ties with Russia after his two-hour call with Vladimir Putin on Monday.

The U.S. leader was upbeat in his summary of the call, saying it went 'very well', while the Kremlin stressed the warm nature of the conversation.

The pair addressed each other by their first names and Putin congratulated Trump on the birth of his grandson, Kremlin foreign aide Yuri Ushakov told reporters.

Trump said: 'Vladimir, you can pick up the phone at any time, I will be happy to answer, I will be happy to talk to you,' Ushakov said.

But despite the cordiality, neither side came away from the call with a clear timeline for a Ukraine ceasefire - though Trump said talks would begin 'immediately'.