Ukraine's government has ordered the expulsion of Russia's military attache, saying he had been caught red-handed receiving classified documents related to the country's cooperation with NATO.

The unnamed attache was taken into custody on Wednesday, has been declared persona non grata and will be thrown out of Ukraine, officials say.

"On April 30, he was caught red-handed receiving classified material from his source," said Maryna Ostapenko, a spokeswoman for Ukraine's security service, the SBU.

The incident comes a day after acting President Oleksandr Turchynov acknowledged that his forces are either "helpless" to stop pro-Russia separatist rebels or actively colluding with them in the seizure of eastern cities.

At dawn on Wednesday, gunmen took control of the towns of Horlivka and Alchevsk after key buildings in the capital of the easternmost province, Luhansk, were seized the day before.

Turchynov said his government's goal is to prevent the spread of unrest ahead of presidential elections scheduled for May 25.

Meanwhile, German Chancellor Angela Merkel has asked Russian President Vladimir Putin for his assistance in securing the release of seven observers from the Organization for Security and Cooperation in Europe, who have been held by pro-Russia separatists in Ukraine.

"The German chancellor reminded President Putin of Russia's responsibility as an OSCE member state and called on the president to exert his influence," Merkel spokeswoman Christiane Wirtz said.

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