Ukraine on Saturday sought concrete steps from Russia to back up a tenuous truce it extended with pro-Moscow rebels in the hope of calming a deadly insurgency sparked by its new westward course.

President Petro Poroshenko returned triumphant from Brussels on Friday having opened the way to Ukraine's eventual membership in the European Union by signing the final chapters of a landmark free trade and political association accord.

A new round of Western sanctions on Russia over the crisis in Ukraine could seriously impact its already stalled economy, Economy Minister Alexei Ulyukayev said Saturday.

The minister said Russia has prepared for three possible scenarios in the event of tougher economic sanctions.

Some 110,000 people have fled to Russia from Ukraine while more than 54,000 have been displaced inside the conflict-torn country, the U.N. said Friday.

"Since the start of 2014, 110,000 Ukrainians have arrived in Russia," Melissa Fleming, spokeswoman for the U.N.'s refugee agency, told reporters.

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.

The rights group running Russia's only museum in a preserved former Soviet labor camp warns that the facility is facing closure as authorities grow increasingly hostile to probing the country's totalitarian past. 

The Perm-36 museum -- named after the notorious prison camp where it is housed -- has seen operations grind to a halt after local authorities cut off key funding "without explanation", the non-governmental organisation Memorial said.

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.