KIEV, Sept 4 (Reuters) - Ukraine's army is on heightened alert near the port of Mariupol on the Sea of Azov near the Russian border, a Ukrainian military source said on Thursday, amid rumours that pro-Russian rebels are advancing towards the city.

"All our divisions are on a high state of alert and are preparing to respond to an attack," the source said when asked about a possible offensive by the separatists on the city.

(Reporting by Pavel Polityuk, writing by Gareth Jones)

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