U.S. Secretary of State John Kerry on Wednesday warned Russia that now was a critical time to implement a fragile peace deal to end the fighting in Ukraine, after his talks with President Vladimir Putin raised hopes of a slackening in tensions.

Kerry said there was an "enormous moment of opportunity" to bring to an end over a year of fighting in eastern Ukraine between government forces and pro-Russia separatists which has dragged relations between Moscow and the West to a new post-Cold War low.

Syrian opposition groups insisted on regime change as they met in Turkey on Wednesday, a day after President Bashar Assad decreed an amnesty for political prisoners following two months of bloodshed.

The three-day gathering -- titled "Conference for Change in Syria" -- opened with the Syrian national anthem and a minute of silence for "the martyrs" killed in bloody crackdowns on street protests simmering in Syria since March.