The Trump administration said it was optimistic about the possibility of a ceasefire in the Ukraine conflict even as President Vladimir Putin appeared to reject US calls for immediate halt to the violence.

Russia struck a sedate tone on Friday but acknowledged grounds for cautious optimism, after only backing the "idea" of a U.S. brokered 30-day ceasefire with firm caveats.

Vladimir Putin said Ukraine must surrender after President Donald Trump urged the Russian leader to spare the lives of Ukrainian soldiers, following ceasefire talks between the U.S. and Russia in Moscow.