Ukrainian government forces came under artillery fire late on Saturday near the strategic port of Mariupol in eastern Ukraine, in the first serious violation of a ceasefire declared only about 30 hours earlier.
The presidents of Russia and Ukraine had earlier said the ceasefire, part of a peace road-map intended to end five months of conflict, was largely holding across eastern Ukraine, where Kiev's forces have been battling pro-Russian separatists.
A Reuters reporter heard prolonged artillery fire late on Saturday to the east of Mariupol, a key flashpoint in the past week of fighting, and then saw a truck, a gas station and an industrial facility ablaze within the limits of the city.
The reporter also saw trucks rushing eastwards conveying agitated-looking pro-government militia volunteers. Tanks and armoured personnel carriers also headed in the same direction.
"There has been an artillery attack. We received a number of impacts. We have no information about casualties," a Ukrainian officer told Reuters.
The area had previously been quiet since the ceasefire took effect on Friday evening, but many residents and combatants in Mariupol and in Donetsk, the other main flashpoint in the conflict, have expressed doubts that the ceasefire can last.