The Organisation for Security and Cooperation in Europe says it lost contact on Thursday with a group of five monitors in separatist eastern Ukraine. Another four-member team has been held by eastern rebels since Monday.
Wolfgang Ischinger, the OSCE's negotiator on national dialogue in Ukraine, told German broadcaster ZDF this week that the monitor mission might have to withdraw if the organisation fears for its employees' lives.
Confrontations continued between government troops and the rebels, who have seized administrative and police buildings across the east and want to join the region to Russia.
At a government-controlled checkpoint in the city of Slovyansk, which has been the epicentre of the conflict, troops came under fire but repelled the attack without casualties. There have been daily attempts in recent weeks to break the army's ring of checkpoints around the city.
As violence escalated in the region, the Ukrainian and pro-Russian sides have blamed each other for the rising number of civilian casualties in the conflict.