MOSCOW (CNN) -

Ukraine's government and separatist leaders signed a ceasefire deal Friday after talks in Belarus, raising hopes of an end to the nearly five-month conflict that has wracked eastern Ukraine.

The ceasefire went into effect at 6 p.m. local time (11 a.m. ET) on the order of Ukrainian President Petro Poroshenko.

It was not immediately clear if it would hold.

Artillery fire and explosions were heard in the eastern Ukrainian city of Donetsk as of 6 p.m. local time, the city's website said.

But a CNN team in southeastern Ukraine, where Ukrainian forces and the rebels have engaged in fierce fighting this week between the Russian border and the port city of Mariupol, said that as of 20 minutes into the ceasefire period there had been no renewal of artillery fire.

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.

"The entire world strives for peace, the entire Ukraine strives for peace, including millions of Donbas residents," he said.

"The highest value is human life. We must do everything possible and impossible to terminate bloodshed and put an end to people's suffering."

A previous unilateral ceasefire declared by the Ukrainian government in June broke down after 10 days.

Sanctions on table

A ceasefire deal may have been signed, but Putin remains under international pressure over Russia's actions in Ukraine.

UK Prime Minister David Cameron, who is hosting the summit in Wales, said in his final remarks that what Putin was doing is "indefensible and wrong."

A package of EU sanctions "is being finalized in Brussels that will further increase the economic costs to Russia for its behavior," he said.

"We stand firmly behind Ukraine's right to make its own decisions, not to have then dictated by Russian tanks rolling over the border."

Europe and the United States are firmly resolved to maintain their pressure on Russia, in support of Ukraine, he said.