Barely 30 hours after a ceasefire came into effect in eastern Ukraine, government forces came under prolonged artillery fire late on Saturday.
Video footage shows a petrol station and an industrial facility ablaze in the strategic port of Mariupol.
The ceasefire terms were agreed in Minsk on Friday, intended to end five months of conflict with pro-Russian separatists.
The area had previously been quiet since the
truce took effect.
The threat of Mariupol enabling a land corridor between Russia and Crimea prompted Ukrainian forces to try to repel a major offensive by pro-Moscow rebels in the days before the truce.
In the other main flashpoint – Donetsk, which is under rebel control, residents described sporadic shelling and gunfire on Saturday, especially near the government controlled airport.
The latest violence by pro-Russian separatists is likely to fuel concerns that the truce maybe a delaying tactic.
Days before, Ukraine accused Russia of sending troops and arms onto its territory in support of the rebels.