Shelling and shots fired in Mariupol, with Kiev and rebel forces denying responsibility after truce was agreed at talks in Minsk

There was shelling and gunfire in east Ukraine on Sunday as a fragile ceasefire agreed on Friday was broken, with both sides accusing the other of violating the truce. One woman was killed overnight in the city of Mariupol, which is still in Ukrainian hands but braced for assault from rebel forces.

Details of the peace plan agreed during talks in Minsk, Belarus, between Kiev and the restive eastern regions of Luhansk and Donetsk were also made public, by the Organisation for Security and Co-operation in Europe (OSCE), who took part in the talks.

Kiev and separatist rebels signed up to a 12-point plan that includes a ceasefire, exchange of all prisoners, and "decentralisation" of power for the eastern regions. It appears to suggest that Donetsk and Luhansk will remain inside Ukraine, although representatives of the separatist governments claim they are still seeking full independence from Kiev. Analysts have speculated that Russia may be interested in setting up a statelet in east Ukraine that is technically ruled by Kiev but has so much autonomy it can essentially be ruled as a Russian protectorate.

The prisoner swap was expected imminently but neither side was forthcoming with details of how and when it would happen.

The ceasefire held for much of Saturday, but overnight there was heavy shelling on the outskirts of Mariupol. There were some reports that Grad rockets were used by the pro-Russia forces but later, sources among the fighters defending the city said there had been artillery strikes but not rockets used.

The official Twitter account of the Donetsk rebels announced as the shelling was going on that its forces were "taking Mariupol", but later said the rebels had merely been responding to Ukrainian "provocations". There were also blasts audible in Donetsk, where rebels control the city but the Ukrainian government controls the airport. Again, both sides accused the other of firing.

The rebels control the city of Novoazovsk, close to the border with Russia, which was seized two weeks ago by rebels apparently backed by soldiers and armour from the Russian army. From there, they have been advancing towards Mariupol, and fierce fighting on Friday before the ceasefire saw casualties on both sides.

Kiev says Russia entered the conflict more decisively as the rebels were facing a defeat, and turned the tide of the conflict. Moscow has denied all reports of its military activity in Ukraine and said any soldiers fighting there are either "lost or on holiday".

City authorities said a 33-year-old woman had been killed in the shelling, however witnesses reported seeing a woman dying after the Lada she was travelling in came under gunfire well inside the city boundaries. It appeared to be the same woman that authorities claimed had died in shelling. It was unclear who had opened fire, though the shooting happened well inside territory controlled by Ukrainian army and volunteer forces.

Amnesty International said over the weekend that both Ukrainian and separatist militias have been responsible for war crimes, and also accused Russia of "fuelling" separatist crimes.

"All sides in this conflict have shown disregard for civilian lives and are blatantly violating their international obligations," said Salil Shetty, Amnesty secretary general.