DONETSK, Ukraine - US-led military exercises involving 15 countries were set to begin in Ukraine Monday, as fighting rumbles on in the restive east between government forces and pro-Russian rebels in violation of a ceasefire.
Heavy fighting erupted around the rebel stronghold of Donetsk in eastern Ukraine on Sunday, piling further pressure on a precarious nine-day-old truce between the government and separatist fighters.
Large clouds of thick black smoke billowed over the industrial city as the boom of sustained shelling and the rattle of automatic gunfire rang out throughout the day, AFP reporters witnessed.
Donetsk council said there had been civilian casualties and described the situation in the city as "critical" but gave no further information.
Kiev accused the rebels of jeopardising the truce by intensifying their attacks on government positions in eastern Ukraine, the scene of five months of deadly combat.
While Kiev and the West accuse Russia of playing a role in the rebellion in the east, soldiers from the 15 nations will begin military exercises dubbed "Rapid Trident 14" near Lviv in the west of the country.
The United States was expected to send around 200 troops to take part in the exercises, the Pentagon said earlier this month, in a show of solidarity with Kiev.
It will mark the first deployment of US ground troops to Ukraine since the Kiev government's conflict with pro-Russia separatists erupted earlier this year.
Sunday's fighting in the east appeared to be concentrated near Donetsk airport where the Ukrainian military said it had driven back an assault by insurgent fighters on Friday.
"The terrorist actions are threatening the realisation of the Ukrainian president's peace plan," said security spokesman Volodymyr Polyovy.
He also took aim at comments by two rebel leaders who both signed the 12-point truce deal in Minsk on September 5, but who declared on Sunday they were mere "observers" at the talks.
The ceasefire is seen as a first step in efforts to draw up a longer term peace deal to end a conflict that has cost more than 2,700 lives and set off the worst crisis in East-West relations since the Cold War.
President Petro Poroshenko has pledged to offer the eastern regions that form the economic backbone of Ukraine some limited self-rule, although the separatists say they want nothing less than full independence.
Rebels and government forces swapped dozens of captives also on Sunday in the latest exchange agreed under the accord, but the insurgents said Kiev's forces were still firing at them.