By Gabriela Baczynska and Aleksandar Vasovic
DONESTSK/MARIUPOL, Ukraine, Sept 6 (Reuters) - An uneasy calm prevailed in eastern Ukraine on Saturday after Ukrainian forces and pro-Russian separatists signed a ceasefire as part of a drive to end a war that has triggered a deep crisis in relations between Russia and the West.
The peace roadmap, approved by envoys in Minsk on Friday, includes the exchange of prisoners-of-war. A separatist leader said this process would begin later on Saturday, though the Ukrainian side said details were still being worked out.
The two sides remain far apart on the future status of the rebel-held areas in eastern Ukraine and both residents and combatants said they did not expect the ceasefire to last long, but there were no reports of serious violations on Saturday.
"The forces of the anti-terrorist operation support the ceasefire and are closely observing the order of the commander-in-chief," the spokesman for Ukraine's National Security and Defense Council, Andriy Lysenko, told a daily briefing in Kiev.
In rebel-held Donetsk, the region's industrial hub with a pre-war population of about one million, separatist commanders said they did not believe the five-month war was over.
"The ceasefire is looking good for now but we know they (the Ukrainian side) are only using it to bring in more forces here and ammunition and then to hit us with renewed strength," said one rebel commander known by his nickname Montana.
"Come what may, I would not trust (Ukraine's President Petro) Poroshenko. And it's not him making the call anyway but the Americans and that is even worse."
Poroshenko agreed to the ceasefire after Ukraine accused Russia of sending troops and arms onto its territory in support of the separatists, who had suffered big losses over the summer. Moscow denies sending troops or arming the rebels.
"I am sure that Ukraine as a state and I as leader of that state are doing everything possible to achieve peace in our country," Poroshenko said in an interview for the BBC's 'Hard Talk' program broadcast late on Friday.
He was speaking after attending a two-day NATO summit in Wales at which U.S. President Barack Obama and other leaders urged Russia to pull its forces out of Ukraine. NATO also approved wide-ranging plans to boost its defenses in eastern Europe in response to the Ukraine crisis.
Obama said he was skeptical that the separatists in eastern Ukraine would deliver on their ceasefire obligations.
The European Union announced new economic sanctions against Russia late on Friday over its role in Ukraine but said they could be suspended if Moscow withdraws its troops and observes the conditions of the ceasefire.
Russia's foreign ministry responded angrily on Saturday to the measures, pledging unspecified "reaction" if they were implemented. Moscow responded to a previous round of U.S. and EU sanctions by banning most Western food imports.
The prime minister of the rebels' self-proclaimed "Donetsk People's Republic", Alexander Zakharchenko, said his side would hand over its POWs to Ukraine on Saturday.
"We hope that on Monday Ukraine will hand over its POWs," he was quoted by Russian news agencies as saying in Moscow.
Ukraine's Lysenko said his side wanted the exchange to take place "as fast as possible" but gave no timeframe. He said the rebels were holding more than 200 Ukrainians captive.
The peace deal, approved in Minsk by envoys from Ukraine, the separatist leadership, Russia and Europe's OSCE security watchdog, also envisages the creation of a humanitarian corridor for refugees and aid.
Before the ceasefire, fighting had raged for days on the outskirts of Donetsk, especially near the airport, which remains in government hands, and also around the port of Mariupol on the Sea of Azov, where government forces have been trying to repel a major rebel offensive Kiev says was backed by Russian troops.
All was quiet on Saturday in and around Mariupol, whose port is crucial for Ukraine's steel exports.
"Many of my men had their first good sleep in days," said one Ukrainian army officer. "I certainly slept well."
In Donetsk, some residents complained of sporadic shelling overnight.
"I don't know what ceasefire we are talking of if there was shooting again. This is no ceasefire but a theater," said Donetsk resident Ksenia.
"This war will go on for five to nine years. Slavs are killing Slavs, there can be nothing worse than that." (Writing by Gareth Jones; Editing by Andrew Heavens)