STORY HIGHLIGHTS
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.
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Ukraine and rebels sign ceasefire deal