US Secretary of State John Kerry on Wednesday said that it was a critical time for Russia to fulfill a fragile peace deal to end the fighting in Ukraine, after he held several hours of talks with President Vladimir Putin.

US Secretary of State John Kerry on Wednesday said that it was a critical time for Russia to fulfill a fragile peace deal to end the fighting in Ukraine, after he held several hours of talks with President Vladimir Putin.

Kerry said at a meeting of NATO foreign ministers in the Turkish city of Antalya that now is a "critical moment for action by Russia and separatists to live up to the Minsk agreement" signed in the Belarus capital earlier this year.

He added that it was also "critical" for observers from the Organization for Security and Cooperation in Europe (OSCE) to be allowed into the conflict areas to monitor the truce.

Kerry had met Putin on Tuesday for the highest level US visit to Russia since the Ukraine conflict erupted in 2013, in a possible sign of a cautious thaw between the two sides.

Kerry said the United States would prefer not to keep sanctions in place against Russia but would keep the measures for as long as required to ensure peace.

He said there was "unanimity" on the part of NATO members on the importance of the Minsk agreement being fulfilled by Russia.

"This is an enormous moment of opportunity for the conflict... to find a path of certainty and resolution," said Kerry, expressing hope that Putin would fulfill his obligations.