Tensions are escalating in eastern Ukraine.

This latest attack comes as armed separatists seize a bus carrying seven international mediators, five Ukrainian army personnel and a bus driver in Sloviansk.

(SOUNDBITE (Ukrainian) FOREIGN MINISTRY SPOKESMAN YEVHAN PEREBYINYS, SAYING:(1)

"(There is no contact) with the members of the military verification mission of the OSCE which is present in Ukraine according to the Vienna document. According to preliminary information they may have been captured by terrorists. This information is being checked now. If it is confirmed then it's a harsh crime committed by terrorists."

Pro-Russian leaders in Sloviansk said they had been held because they were believed to have a spy amongst them from the pro-Western government in Kiev.

(SOUNDBITE) (Russian) SELF-PROCLAIMED MAYOR OF SLOVIANSK, VYACHESLAV PONOMARYOV, SAYING: (2)

"I've heard, I got reports that there was somebody there from the military headquarters. It alrea