VIENNA: The OSCE's acting chairman, Polish Foreign Minister Zbigniew Rau, on Thursday slammed Russia's invasion of Ukraine as a "crime against humanity".

"We condemn in the strongest possible words this act of aggression," he told a meeting, saying Russia justifying "this crime against humanity is deplorable and shameful".

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Rau was speaking at a meeting of OSCE parliamentarians before the organisation convened a closed-door meeting of its permanent council to discuss the security situation in Ukraine.

Poland currently holds the rotating chair of the Vienna-based Organization for Security and Cooperation in Europe, which has 57 member states, including Russia, Ukraine and the United States.

Since 2014, the organisation has been monitoring the east Ukraine conflict with hundreds of observers currently on the ground.

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OSCE Secretary General Helga Schmid said earlier this week that the mission continued to implement its mandate "without interruption".

But amid the rising tensions some members, including the US and the UK, have ordered the withdrawal of their personnel in recent weeks, citing safety concerns.