KIEV, Sept. 7 - A woman died and at least four people were wounded when fighting flared again in eastern Ukraine overnight into Sunday, jeopardizing a ceasefire struck less than two days earlier between Ukrainian government forces and pro-Russian separatists, Reuters reported.

The accord, brokered by envoys from Ukraine, the separatist leadership, Russia and Europe's OSCE security watchdog, is part of a peace plan intended to end a five-month conflict that has killed nearly 3,000 people and caused the sharpest confrontation between Russia and the West since the Cold War.

Shelling resumed near the port of Mariupol on the Sea of Azov late on Saturday, just hours after Russian President Vladimir Putin and Ukrainian counterpart Petro Poroshenko had agreed in a phone call that the truce was holding.

Fighting also broke out early on Sunday on the northern outskirts of rebel-held Donetsk, the region's industrial hub. A Reuters reporter saw plumes of black smoke filling the sky near the airport, which has been in the hands of government forces.

The two cities then turned quiet for much of Sunday, but a Reuters witness in the early evening reported several mortar blasts within the city confines of Donetsk. They damaged a bridge where the rebels had erected a roadblock.

In a new report on the conflict, Amnesty International accused both the rebels and Ukrainian militia of war crimes and it published satellite images it said showed a build-up of Russian armor and artillery in eastern Ukraine.

"Our evidence shows that Russia is fuelling the conflict, both through direct interference and by supporting the separatists in the east. Russia must stop the steady flow of weapons and other support to an insurgent force heavily implicated in gross human rights violations," Amnesty's secretary-general, Salil Shetty, said in a statement.

Moscow denies dispatching forces or arming the rebels despite what NATO says is overwhelming evidence to the contrary.

Both the rebels and the Ukrainian military insisted on Sunday they were strictly observing the ceasefire and blamed their opponents for any violations.

"As far as I know, the Ukrainian side is not observing the ceasefire. We have wounded on our side at various points. We are observing the ceasefire," Vladimir Antyufeyev, deputy premier of the self-proclaimed "Donetsk People's Republic", told Reuters.

Earlier, government forces said they had come under artillery fire east of Mariupol, a crucial port for Ukrainian steel exports. In the days before the ceasefire, they had been trying to repel a big rebel offensive against the city.

The shelling in Mariupol claimed the first civilian casualty since the ceasefire began. Local officials confirmed the death of a 33-year-old woman early on Sunday and said at least four other people had been wounded.

"They, terrorists, Russians, are trying to scare us. They have no respect for the ceasefire. They are lying all the time. They are people with no honor," said Slavik, a Ukrainian soldier armed with a machinegun.

A Reuters reporter at the scene, a few km (miles) from the center of the city of 500,000, saw fires raging just before midnight on Saturday as Ukrainian reinforcements raced east toward the demarcation line separating the two sides.

Poroshenko agreed to the ceasefire after Ukraine accused Russia of sending troops and arms onto its territory to bolster the separatists after they suffered heavy losses over the summer to a Ukrainian government offensive.

The peace roadmap agreed on Friday includes an exchange of prisoners of war and the establishment of a humanitarian corridor for refugees and aid. There was no sign of progress on either plan on Sunday. (rt/ez)