Contact has been lost with a team of European military observers monitors in volatile eastern Ukraine, the Organization for Security and Cooperation in Europe said Tuesday.
The news from the European security watchdog came as Ukraine's military fought fierce battles with pro-Moscow separatists in the eastern city of Donetsk.
The OSCE said its four-member team was on a "routine patrol" east of Donetsk when contact was lost Monday evening at around 6:00 p.m. local time (10:00 a.m. EST).
"We have been unable to re-establish communication until now," the security watchdog said in a statement. "We are continuing with our efforts and utilizing our contacts on the ground."
Both the Ukrainian government and regional authorities are aware of the situation, the statement added.
There was no immediate claim of responsibility but rebel groups have previously kidnapped OSCE monitors in Ukraine, according to the Reuters news agency.
So far, Ukraine's military forces have had little success against rebels who have declared independent "people's republics" in two provinces of the eastern industrial heartland where about 20 people have been killed in recent days
Russia's foreign ministry has urged Kiev to halt what it called "military operations against its own people" and said it wanted the OSCEto investigate clashes with pro-Russian separatists in Donetsk.
The OSCE deployed civilian monitors across Ukraine in March to help try to defuse the crisis in the country's east.
Reuters contributed to this report.