Nine Red Cross workers were released on Saturday by pro-Russian separatists in eastern Ukraine, a day after the eight Ukrainians and one Swiss national were detained, the aid group said.

“Now they will take up their humanitarian work and prepare a medical evaluation,” International Committee of the Red Cross spokesman David Pierre Marquet said.

Their capture in the volatile eastern province of Donetsk occurred as security forces clashed with armed pro-Russian separatists in the port city of Mariupol near the Russian border.

Local authorities said seven people were killed and about 50 injured.

The government in Kiev said security forces killed about 20 pro-Russian militants.

Kiev has sent in forces in a bid to reassert control of the restive east, inflaming the pro-Russian faction. On Saturday, the National Guard withdrew from central Mariupol, which was largely calm.

About 3 million residents of Donetsk and the province of Luhansk have been asked to vote Sunday on whether they want to stay part of Ukraine.

The vote comes after a March referendum organized by pro-Russian authorities in Crimea led to its annexation by Moscow.

That vote has not been recognized by Kiev or internationally.

Russian President Vladimir Putin on Friday visited the Crimean port Sevastapol to mark the anniversary of the 1945 Soviet victory over Nazi Germany.

The government in Kiev, NATO and the United States condemned his visit as a provocation.

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