The news came just two days after four members of the UN monitoring mission for the Colombian peace process were sacked for dancing with FARC rebels at a New Year's Eve party.
It was not immediately clear how many people were involved in previously unseen footage now being investigated and images on social media apparently of a similar nature, the UN source said.
"It is under investigation. And the UN mission in Colombia will take whatever steps may be necessary. We are totally committed to being impartial," the source told AFP.
UN monitors are overseeing the FARC's disarmament as part of a peace deal the leftist guerrillas signed with the government to end a more than five-decade conflict.
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But controversy erupted Thursday when Colombian media broadcast videos of UN monitors, dressed in their pale blue uniform vests, swaying to tropical beats with FARC fighters in their arms at a camp in northern Colombia where the rebels are gathering before laying down their weapons.
The UN said that after investigating the incident, it had decided to "separate" three monitors and their supervisor from its mission.
It said their behavior was "inappropriate" and "does not reflect the mission's values of professionalism and impartiality."
After signing a peace deal with the government in November, the FARC's 5,700 fighters are currently gathering near 26 zones where they are due to demobilize over a period of six months.
The UN has sent 280 monitors to oversee the process, a contingent set to eventually number 450.
Founded in 1964, the Revolutionary Armed Forces of Colombia (FARC) is the largest rebel group in a conflict that has claimed more than 260,000 lives.
