Witnesses in the Ukrainian port city of Mariupol are reporting sustained explosions outside the city and a volunteer battalion of Ukrainian fighters says Grad rockets are being fired at its positions.
The reports tonight come little more than a day after Ukraine and Russian-backed separatist rebels signed a ceasefire after more than four months of fighting in the country's east.
The ceasefire had appeared to largely been holding during much of the day.
But late on Saturday, witnesses in Mariupol told The Associated Press by telephone that heavy explosions were coming from the city's eastern outskirts, where Ukrainian troops retain defensive lines against the rebels.
The volunteer Azov Battalion said on Facebook that their positions were hit by Grad rockets, but did not give details.