The Reuters news agency early on Sunday said an industrial facility, a lorry and a petrol station were set ablaze in an area within the limits of Mariupol, the scene of fierce fighting up until a ceasefire was agreed on Friday.
Reuters described the attack as "prolonged". "There has been an artillery attack. We received a number of impacts, we have no information about casualties," a Ukrainian officer told the agency at the scene.
John Wendle, reporting for Al Jazeera from near Mariupol, said heavy calibre machine guns had also been heard near the city, and he had unconfirmed reports of civilian casualties. He said pictures he had seen showed civilian cars destroyed in the attack. Reuters also reported that lorries carrying pro-government militia volunteers and tanks and armoured personnel carriers were seen heading towards the site of the attack.
The city of 500,000 people on the Sea of Azov near the Russian border had been the scene of fierce fighting between Ukrainian government forces and pro-Russian separatists before the ceasefire deal. Earlier Saturday, the presidents of Ukraine and Russia said the ceasefire was mostly holding. A statement from the Ukrainian president, Petro Poroshenko, said he and Russian counterpart Vladimir Putin discussed steps "for giving the ceasefire a stable character'' in a telephone conversation Saturday.
Europe's OSCE security watchdog approved the ceasefire in Minsk on Friday as part of a peace agreement that also includes an exchange of prisoners of war and establishing a humanitarian corridor for refugees and aid.