The U.N. had no details on casualties but Saye Gornleh, a reporter for Liberia's independent Star radio station, said in a broadcast that he'd seen ambulances take the bodies of two crew members from the burning chopper in the town of Dagamo in Nimba County.
The nationalities of those aboard was not immediately known.
"We are organizing a rescue mission," said Ben Malor, spokesman for the 15,000 strong peacekeeping mission in the West African country. He said it was not known what caused the crash.
Malor said the aircraft was on a routine flight from Ganta on the border with Guinea and was heading to Loguatuo on the border with Ivory Coast.
The U.N. has had peacekeepers in Liberia for years, helping maintain security since the nation's civil war ended in 2003.