Ukraine, rebels sign ceasefire deal as EU leaders consider sanctions on Russia

President Barack Obama said he was hopeful but skeptical that a ceasefire in Ukraine would hold, questioning if pro-Russian rebels would adhere to it.

Obama, speaking at the end of a NATO summit in Wales, added that NATO was "fully united in support of Ukraine's sovereignty, independence, territorial integrity and ability to defend itself."

Member nations would send nonlethal military aid and help modernize Ukraine's security forces while the United States and European allies finalize measures "to deepen and broaden sanctions" against Russia, he said.

"Russia's aggression against Ukraine threatens our vision of a Europe that is whole, free and at peace." he said.

Speaking to reporters at the same summit, Poroshenko said the ceasefire deal was based on his peace plan and an agreement reached in a phone call this week with Russian President Vladimir Putin.

He said the sovereignty, territorial integrity and independence of Ukraine were key to the 12-point plan. He also said he hoped the exchange of prisoners would start in the near future, perhaps as soon as Saturday.

Poroshenko said it was important that ceasefire lasted, and that during this period a political dialogue should continue on restoring peace and stability to the Luhansk and Donetsk regions.

"We are ready to provide significant steps, including the decentralization of power," he said, as well as greater economic freedoms for the Luhansk and Donetsk regions and guarantees that their culture and language be respected. Many in the region are Russian speaking.

The self-styled Donetsk People's Republic Twitter feed also said the ceasefire had been signed and that it would come into effect at 6 p.m. local time.

However, the ceasefire does not mean the end of the Donetsk People's Republic and the Luhansk People's Republic, separatist leaders said at a televised news conference after signing it.

Poroshenko has asked his foreign minister and the Organization for Security and Cooperation in Europe, which already has international observers in Ukraine, to monitor compliance with the ceasefire.