Ukrainian President Petro Poroshenko on Thursday accused Russia of failing to fully back a peace plan to end the bloodshed in the east of his country.

The plan "will only be able to work if Russia cooperates. Up to now unfortunately the support (from Moscow) has been insufficient," Poroshenko told the Parliamentary Assembly of the Council of Europe in Strasbourg.

The United States warned Russia on Thursday it had only "hours" to prove it was helping disarm Ukrainian insurgents whose separatist drive has reopened a Cold War-style chasm in East-West ties.

U.S. Secretary of State John Kerry's warning came a day before Ukraine's 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.

U.S. President Barack Obama warned Wednesday that additional sanctions would be in store if Russia does not move swiftly to reduce tensions in eastern Ukraine.

In a telephone call with Italian Prime Minister Matteo Renzi, Obama also vowed to press Russia to persuade separatist groups to abide by a ceasefire agreement, the White House said.

Russia must take "concrete" steps to resolve the Ukraine crisis, U.S. Secretary of State John Kerry said Wednesday, warning Moscow could easily reinstate its parliamentary approval for military intervention.

Russian President Vladimir Putin must publicly call for pro-Moscow rebels to lay down their arms and cut support to them, Kerry said, adding: "There are many concrete things that would make a difference on the ground."

Dawn had barely broken when shelling began in the rebel bastion of Slavyansk, a ghost town emptied of half its people and besieged by Ukrainian government forces.

For 20 minutes early Wednesday the two sides traded fire, oblivious to the week-long truce ordered by Ukraine's new president and backed by a top leader of the pro-Russian separatists.

NATO head Anders Fogh Rasmussen said Wednesday there has been no change in Russia's behavior over Ukraine, charging that Moscow had "broken the rules" and eroded international trust.

Since the Cold War ended more than 20 years ago, NATO had tried to build a working, practical relationship with Russia but all that had now come to nothing, said Rasmussen.

Russia's upper chamber of parliament on Wednesday voted to scrap an earlier resolution allowing President Vladimir Putin to send troops into Ukraine, in a move Moscow says will help the peace process.

Only one senator voted against Putin's Tuesday proposal to rescind the March 1 decision granting him the right to intervene in Ukraine to protect Russian speakers, while 153 voted in favor.

Ukrainian Foreign Minister Pavlo Klimkin Wednesday vowed that Kiev will stick by its unilateral ceasefire despite the downing of an army helicopter by pro-Russia separatists.

"We stick to our commitment, we stick to our unilateral ceasefire," Klimkin told reporters at NATO, after nine servicemen were killed in Tuesday's attack outside the city of Slavyansk.

The case for tougher sanctions against Russia gets stronger every day Moscow fails to match its welcome for the Ukraine government's peace plan with concrete action, British Foreign Secretary William Hague said Wednesday.

The shooting down of a Ukrainian helicopter, killing all nine on board, by pro-Russian rebels Tuesday, was hard to reconcile with President Vladimir Putin's backing of the plan, Hague said.

Russia on Wednesday said it hoped Kiev and the international community will heed the "positive signals" it was sending over the Ukraine crisis after President Vladimir Putin moved to scrap the option to invade. 

"We are counting on the positive signals that the Russian president is now sending being heard across the world and, above all, in Ukraine," Deputy Foreign Minister Grigory Karasin told Russian news agencies.