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By Anton Zverev

DONETSK, Ukraine, Aug 28 (Reuters) - Pro-Moscow rebel forces in Ukraine gained a foothold on the Sea of Azov on Thursday and their objective is to fight their way down the coast to the Ukrainian port city of Mariupol, the leader of the main separatist entity said.

Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic, told Reuters in an interview that about 3,000 Russian volunteers were serving in the rebel ranks.

"Today we reached the Sea of Azov, the shore, and the process of liberating our land, which is temporarily occupied by the Ukrainian authorities, will keep going further and further," Zakharchenko said in the city of Donetsk, the main rebel stronghold in eastern Ukraine.

He said the new front in the south along the Sea of Azov coast was "economically, militarily and politically the only front on which we should place our emphasis."

"Taking Mariupol, the second biggest town in Donetsk region, will allow us to expand our units by another five or seven thousand," he said, referring to the port city.

Mariupol sits across the Sea of Azov from Crimea, which Russia annexed from Kiev in March. The city has been the headquarters of pro-Kiev local authorities of the Donetsk region forced out from the regional capital by the rebels.

Ukrainian President Petro Poroshenko said on Thursday Russian forces had invaded Ukraine.

But Zakharchenko said: "There were and are volunteers from Russia."

"They come from all corners, not just of Russia; we have Serbs fighting, we have Frenchmen fighting, Turks fighting, even an Italian with a Ukrainian residency permit."

He said the Russians fighting with the rebels come "from St Petersburg, from the (Russian) far east, from Siberia, Cossacks from the Don region, Kuban, Tver region."

Asked if he wanted official backing from the Russian state, Zakharchenko said: "On the one hand, it would make things much simpler for us. The war would quickly be over. But on the other hand, I understand perfectly that it's not realistic, it's not possible"

"The international community would simply not allow that... To be frank, we are capable of resolving the strategic and tactical objectives on our own. Our offensive demonstrates that clearly," he said. (Reporting by Anton Zverev; Writing by Christian Lowe; Editing by Gabriela Baczynska)

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