Looking for His glorious appearing! Titus 2:13

US Secretary of State John Kerry's ultimatum was delivered a day before Ukrainian President Petro Poroshenko signs the final chapters of an historic EU accord that nudges his country toward eventual membership and pulls it firmly out of Russia's reach.

The West is scrambling to save a temporary ceasefire and nascent peace talks that pro-Russian separatists, who are now threatening the ex-Soviet state's survival, agreed to at the start of the week.

Mr Poroshenko on Thursday pushed back the expiry of the truce, broken on repeated occasions, but still having succeeded in tempering the worst of the violence in the Russified eastern rustbelt, for a few hours until Friday at 7pm (5am AEST).

But Ukraine's new Western-backed leader and separatist commanders have set up a third round of indirect negotiations in the eastern hub of Donetsk that theoretically backs another extension.

The Kremlin said Mr Putin assured German Chancellor Angela Merkel that he fully supports the resumption of meaningful dialogue between the warring sides.

Mr Putin and Ms Merkel discussed "the need to extend the truce, establish regular contact group meetings, and the release of forcibly detained individuals," the Kremlin said in a statement.

Unrest ... demonstrators wear the Ukrainian flag during a protest against Russia's intervention in Ukraine. Picture: AFP PHOTO

But Mr Poroshenko told a European parliamentary assembly in Strasbourg that "up to now, unfortunately, the support (from Russia) has been insufficient".

The 11-week insurgency has killed more than 435 people and shattered the delicate system of trust that developed between Moscow and the West since the Cold War.

Mr Putin is strongly suspected of orchestrating the Ukrainian uprising after having seized its Crimea peninsula in reprisal for the February ouster in Kiev of a Moscow-backed president.

The Kremlin chief denies exerting control over the fighters and is yet to address in public reports from Kiev and Washington of rocket launchers and even tanks crossing the Russian border into the conflict zone.

Mr Kerry stressed in Paris 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."