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Russian Prime Minister Dmitry Medvedev said he had grave doubts about the legitimacy of neighboring Ukraine's new leaders on Monday.

"If we consider government the people who were moving around Kiev wearing black masks and holding Kalashnikov machine guns, then it is going to be very difficult for us to work with such government," he said, according to Russian news agency

Some news agencies quoting Medvedev as saying that Ukraine's government had come to power as a result of “armed mutiny.”

His comments came just hours after Ukraine's interim leader Oleksander Turchinov announced that a new presidential election campaign would start this week following the ouster of President Viktor Yanukovych, a key Russian ally.

"We do not understand what is going on there," Medvedev told local news agencies. "There is a real threat to our interests and to the lives of our citizens. There are big doubts about the legitimacy of a whole series of organs of power that are now functioning there."

Medvedev said that any extension of a key gas agreement between the two nations would have to be negotiated between Ukrainian companies and the government.

"Those agreements which are legally binding must be fulfilled," he said, according to Interfax.

A warrant for Yanukovych's arrest along with several other officials was for the "mass killing of civilians," Interior Minister Arsen Avakhov said on his official Facebook page.

Reuters and The Associated Press contributed to this report.