A Polish military officer released on Saturday with a group of OSCE observers in Kramatorsk, eastern Ukraine, said his team faced "real danger" while being held by pro-Russian militants.

Unmarked soldiers wait on the road as separatists block the Kramatorsk road to prevent them from advancing near Krematorsk, Ukraine, 02 May: photo - EPA/IGOR KOVALENKO

Major Krzysztof Kobielski, now back safely in Poland, told the TVN24 broadcaster: "When there was shooting we didn't speak, we just stayed lying on the floor. All around us there were hundreds of men armed with knives, pistols and automatic weapons."

The 12 OSCE observers were detained by pro-Russian militants in the town of Kramatorsk, south of rebel-held Slaviansk on 25 April.

German Col. Axel Schneider, who led the group, told the Associated Press that the 12 detainees held up well.

"They had a very good attitude and that gave them the strength to stand the situation," Schneider said as the conflict in Ukraine became increasingly deadly.

On Sunday, pro-Russian militants stormed a Ukrainian police station in Odessa and freed nearly 70 activists detained during violent clashes with pro-Ukraine supporters.

The militants chanted "We will not forgive!" and "Russia!" as they smashed windows and broke down the gate at the compound two days after over 40 pro-Russian activists died in a blaze at a trade union building in Odessa which they had occupied, Reuters reports.

A Pro-Russian supporter breaks a window of the offices of Industrial Union of Donbass Corporation in Donetsk, eastern Ukraine, 03 May 2014. According to reports, pro-Russian supporters led by masked men stormed the regional office belonging to the Ukrainian intelligence service (SBU) in Donetsk chanting pro-Russia slogans and shouting 'We will not forgive Odessa' on 03 May. They then moved to Industrial Union of the Donbass and smashed their way into the building. No police were deployed to block the building takeover, reports indicate: photo - EPA/MAYSUN

Acting Ukraine President Oleksander Turchinov blamed lack of action by local police for the deaths.

"In [the Odessa region] there are whole structures working together with the terrorists," Turchinov said in a television interview. "This is a colossal problem."

Two Ukrainian pilots also died after their helicopters were shot down by militants near Slaviansk. (pg)