Ukrainian president Petro Poroshenko has said he is ready to strike a peace deal with Russian President Vladimir Putin that could put an end to a separatist insurgency gripping the ex-Soviet state.

"I am ready to make peace with anybody," Mr Poroshenko told CNN when asked about the possibility of a peace deal with the Russian leader.

"I hate the idea of not to use the last opportunity to bring peace to the region."

Mr Putin earlier told German Chancellor Angela Merkel of the worsening humanitarian crisis in Ukraine, the Kremlin has said.

In a statement following the talks, the Kremlin said that Mr Putin "drew attention to the worsening humanitarian situation and an increase in the number of Ukrainian refugees in Russia".

The leaders discussed "the need to extend the truce" and the observance of the ceasefire by the warring sides in Ukraine, the Kremlin said, adding the call took place at the initiative of the German side.

A one-week ceasefire announced by Mr Poroshenko last week is set to expire tomorrow after inconclusive talks.

Washington has threatened to impose new sanctions against Russia over the Ukraine crisis.

US Secretary of State John Kerry warned earlier today that Russia must prove "in the next hours" that it is working to help disarm separatist groups in restive eastern Ukraine.

He was speaking after meeting with France's foreign minister.

My Kerry said: "We are in full agreement that it is critical for Russia to show in the next hours, literally, that they're moving to help disarm the separatists, to encourage them to disarm, to call on them to lay down their weapons and to begin to become part of a legitimate process."

He added that European Union leaders would discuss possible sanction moves on Russia at their summit in Belgium starting today.

He added: "We all agree that they need to be ready. But our preference is not to have to be (in) a sanctions mode. We would like to see a cooperative effort between the United States, Europe and Russia and the Ukrainians."