The gun battle erupted overnight when around 300 people lobbed petrol bombs and fired weapons at an interior ministry base in the port city of Mariupol, Interior Minister Arsen Avakov said on his Facebook page.

The violence raised the urgency of the Geneva talks, which bring together the foreign ministers of Ukraine, Russia, the United States and the European Union in the hopes of starting dialogue to quell the biggest East-West crisis since the end of the Cold War.

Russia, which has tens of thousands of troops stationed on its border with Ukraine, has warned Kiev not to use force against the pro-Kremlin protestors who have seized buildings across the country's tinderbox southeast and has said it reserves the right to protect Russian speakers there.

The United States warned Moscow on Wednesday that it risked facing fresh sanctions unless it made concessions at the crunch Geneva talks.

The negotiations come a day after a military operation ordered by Kiev to oust the separatists collapsed, with militants showing no sign of budging and humiliating the government by seizing army vehicles originally dispatched to clear them out.

As tensions simmered, NATO announced that it was deploying more forces in eastern Europe, and called for Russia to stop "destabilizing" Ukraine, which has been in deep turmoil since the ouster of pro-Kremlin president Viktor Yanukovych in February.

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