KIEV - Around 40 people had been held captive in Ukraine's eastern town of Sloviansk, where pro-Russia protesters seized government buildings, authorities said Monday.

According to Maryna Ostapenko, spokesperson of the Ukrainian Security Service (SBU), the activists, who had captured the people, had not put forward any requirements to release the hostages.

Ostapenko said Russian Special Forces "might be involved" in the kidnapping of European military observers in the east city of Sloviansk.

On Friday, pro-Moscow separatists seized a bus with 8 international observers, who were operating under the mandate of the Organization for Security and Cooperation in Europe (OSCE), and five Ukrainian military officers escorting them.

The captors claimed to have found a Ukrainian spy travelling with the group.

On Sunday, the rebels freed one of the observers for health's reasons, but said they had no plans to release other international observers and Ukrainian officers.

On Monday, the pro-Russian protest spread in another town in eastern Ukraine as a group of people, including many in masks, seized a government building in Gorlovka in Donetsk region.

Pro-Moscow demonstrators stage their protest in eastern Ukraine since early April, demanding referendum on region's autonomy and closer ties with Russia.