Ukraine crisis: Prisoner swap boosts ceasefire
Ukrainian government and rebel forces have exchanged dozens of prisoners, a week after a ceasefire came into effect in the east of the country.
A total of 139 Ukrainian soldiers were freed and 52 rebels.
The exchange is the first step carried out successfully under the terms of the 12 February Minsk agreements, brokered by France and Germany.
Later on Saturday the rebels reportedly agreed to begin a pullback of heavy weapons, another key part of the truce.
The moves came as the US said it was considering "serious sanctions" against Russia following breaches of the truce.
During a visit to London, US Secretary of State John Kerry said Mr Obama would make a decision on the sanctions in the coming days.
He accused Moscow of "craven behaviour" in its support for the rebels, undermining a ceasefire.
"If this failure continues, make no mistake, there will be further consequences including consequences that will place added strains on Russia's already troubled economy," Mr Kerry said.
But Kremlin spokesman Dmitry Peskov said sanctions would not help solve Ukraine's crisis.
Nearly 5,700 people have died since the fighting erupted last April and some 1.5 million people have fled their homes, according to the UN.
The Ukrainian government, Western leaders and Nato say there is clear evidence that Russia is helping the rebels in eastern Ukraine with heavy weapons and soldiers. Independent experts echo that accusation. Moscow denies it, insisting that any Russians serving with the rebels are "volunteers".
Ukrainian President Petro Poroshenko tweeted the news of the prisoner exchange, which took place near the front line town of Zholobok.
Some of the government soldiers were wounded and walking on crutches.
Mr Poroshenko originally said 140 soldiers would be freed, and Ukrainian officials said one more soldier would be released in the coming days.
The BBC's David Stern in Kiev says the exchange was the first positive news since the Minsk agreements, after a planned ceasefire failed to take hold.
However, the agreements called for a full exchange, and each side reportedly still holds a number of prisoners. It is not clear how many people are still in captivity.
Hours later, a Russian general involved in implementing the ceasefire said rebel forces had agreed to begin withdrawing heavy weapons from the frontline.
Alexander Lentsov said the leaders of the self-styled Luhansk and Donetsk people's republics had signed the orders.
"We have designated 22 February as "D-Day" as determined by the agreement from all sides. So from tomorrow we will within the period of 14 days observe the agreement on the pullback of heavy weapons."
The process had been due to start last Tuesday and completed by 3 March, but Gen Lentsov said it would now take until 7 March.
A Ukrainian official said the government had been informed of the move, but it was not yet clear whether it was being reciprocated.
Ukraine and rebel forces accuse each other of multiple breaches of the truce, and there were reports of violations around Donetsk and the port city of Mariupol on Saturday.
The rebels took the strategic transport hub of Debaltseve during the week in spite of the ceasefire, arguing the truce did not apply to the flashpoint town, forcing government troops to retreat.
Paul Adams, BBC News, Donetsk
From the centre of Donetsk, it's still possible to hear the distant rumble of artillery and rockets. The area around the devastated airport remains, as it has been for months, a flashpoint.
Reports from an area just to the east of the southern port city of Mariupol suggest that shelling continues there too.
But the Organization for Security and Cooperation in Europe, which has monitors on the ground, says it's seeing signs of progress elsewhere.
Mr Kerry described as "unacceptable" the situation around Mariupol, where rebels were accused of shelling government forces and building up equipment and troop numbers.
The Ukrainian government fears that rebels will attempt to capture the city to provide a corridor to Crimea, a Ukrainian territory annexed by Russia last year.
The continued fighting comes as Ukraine marked a year since the downfall of pro-Russian President Viktor Yanukovych, following mass protests in central Kiev.
A rally with an estimated 35,000 participants took place in Moscow on Saturday to condemn the ousting, which many Russians regard as a coup.