MARIUPOL, Ukraine, Sept 4 (Reuters) - A Reuters witness heard about a dozen blasts and saw plumes of black smoke rising a few km (miles) to the east of the port of Mariupol in eastern Ukraine on Thursday.
Earlier, a military source said Ukraine's army was on a state of heightened alert near Mariupol, a strategic port city on the Azov Sea that guards the way towards Russian-annexed Crimea.
A Ukrainian soldier told Reuters he had seen pro-Russian separatists advancing on the city with tanks, armoured personnel carriers and artillery. His comments could not be independently confirmed.
"Our plan is to stay here (in an eastern district of Mariupol) and not to budge an inch," said the soldier, who was from Ukraine's Azov batallion.
(Reporting by Aleksandar Vasovic, writing by Gareth Jones)