The anti-terrorist operation in eastern Ukraine is turning deadly, with three people killed and more than a dozen wounded in a puzzling and senseless attack on a military base denounced by protesters as a "provocation".

The Ukrainian Interior Ministry said its troops late on Wednesday had repelled an attack by pro-Russian forces on a military outpost in Mariupol, a city in Ukraine's southeast.

Three attackers have been killed and 13 others wounded, the Ministry said. This is the worst violence in Kiev's four-day military crackdown on protesters in eastern Ukraine.

Anti-government protesters who control the city hall in Mariupol said they had not authorised the attack. Activists said the attack appeared to have been the work of provocateurs from the fascist Right Sector, who led a crowd of Mariupol residents to the military base and hurled Molotov cocktails inside. When interior troops fired warning shots, the provocateurs opened fire at the crowd, according to a reporter of LifeNews, a Russian news resource, who was at the scene.

The suspicious attack came less than 24 hours before the Foreign Ministers of Russia, the United States, the European Union and Ukraine began a crucial meeting in Geneva in an effort to find a way to resolve the standoff in Ukraine.

Elsewhere in eastern Ukraine the situation on Thursday remained calm, if tense.

Kiev has announced a ban on Russian males aged between 16 and 60 from entering Ukraine. The security service said they had detained 10 Russian "infiltrators" in eastern Ukraine.

Russia's President Vladimir Putin dismissed as "rubbish" accusations that Russian agents were operating in Ukraine.

In a live TV call-in session on Thursday Mr. Putin said the Ukrainian authorities were dragging the country into an "abyss" by refusing to accommodate the concerns of Russian-speaking regions. He branded Kiev's decision to send the army into eastern Ukraine as a "crime" and said the crisis in Ukraine could only be solved through dialogue and compromise in the interests of the Ukrainian people.

Mr. Putin confirmed that he still had a mandate from the Russian Parliament to deploy the armed forces in Ukraine, but expressed the hope for a peaceful settlement of the crisis.

"I really hope that I do not have to exercise this right and that we are able to solve all today's pressing issues via political and diplomatic means," Mr. Putin said.

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