Kerry also spoke about his visit on Tuesday to Sochi in Russia, where he met with Russian President Vladimir Putin and Russian Foreign Minister Sergei Lavrov.

"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 said.

"It is 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, especially Shyrokyne," Kerry said.