Ukraine on Thursday raised hopes of a ceasefire with pro-Moscow rebels during a NATO summit where Britain and the United States urged the international community to stand up to Russia and counter the threat from Islamic State.

President Petro Poroshenko briefed leaders, including U.S. President Barack Obama and German Chancellor Angela Merkel, and said he expected a deal to be signed on Friday "for the gradual introduction of the Ukrainian peace plan."

Islamic State militants have issued a threat to President Vladimir Putin, vowing to oust him and "liberate" the volatile North Caucasus over his support of the Syrian regime.

The General Prosecutor's Office of Russia demanded that access to the address, which was posted on YouTube on Tuesday and features what jihadists say is a Russian-supplied fighter jet, be blocked.

Germany's foreign minister on Wednesday saw a tentative "sign of hope" after a ceasefire announcement by Ukraine but warned that Kiev, and especially Moscow, needed to show they meant it.

Frank-Walter Steinmeier said reports of talks between the Ukrainian and Russian presidents and "cautious" discussion of a ceasefire "is perhaps a small sign of hope" but that the next few days would reveal if it signified more.

France said Wednesday "conditions" were not in place to deliver the first of two Mistral-class warships to Russia, a move planned later this year that has sparked controversy given the crisis in Ukraine.

"The President of the Republic declared that, despite the prospect of a ceasefire which still remains to be confirmed and implemented, the conditions for France to deliver the first warship are not to date in place," Francois Hollande's office said in a statement, on the eve of a major NATO summit.

Russian President Vladimir Putin on Wednesday called on both Ukrainian rebels and government forces to cease fire and agree to the broad terms of a truce ending their four-month war.

Putin's first direct appeal on the insurgents to lay down their weapons came hours after the beleaguered Ukrainian president, Petro Poroshenko, said he and Putin had agreed on a ceasefire deal.

When Russian paratrooper Nikolai Kozlov was deployed to Crimea to help Moscow take over the peninsula from Ukraine, his parents were proud of him.

Six months later Kozlov is disabled for life, an amputee now recovering in hospital from wounds incurred on a Kremlin-orchestrated covert mission to Ukraine to prop up separatists fighting against Kiev.

In thick pine forests hidden in the remote wilderness of eastern Lithuania, young professionals are ditching their suits and ties for camouflage gear, and swapping iPads for rifles.

European Union nations will decide on new sanctions against Moscow by Friday, with Russian aggression towards Ukraine requiring the strongest possible response, incoming EU foreign affairs chief Federica Mogherini said.

Leaders of the 28-member EU decided on Saturday to impose a fresh round of sanctions against Russia after alleging that Moscow had deployed troops and weapons to back a rebel counteroffensive in southeast Ukraine.

NATO leaders plan a powerful show of unity at a summit this week against Russia's alleged aggression in Ukraine, but there is little the military alliance can do to intervene.

The announced deployment of thousands of NATO troops and extra military equipment in Eastern Europe is intended to reassure NATO member states in the former Soviet bloc, but is bound to anger Russia, as it will challenge a key NATO-Russia deal.

Fighting in Ukraine has driven over half a million people from their homes, the U.N. refugee agency said Tuesday, warning that the real number could be double that, in a crisis threatening the entire region.

At least 260,000 have been displaced within Ukraine, UNHCR said, adding that Moscow had reported another 260,000 people have sought asylum in Russia.