The UN mission has been severely hit by the earthquake that levelled much of the Haitian capital, Port-au-Prince.
Scores of UN workers are missing after the mission's headquarters and other buildings collapsed.
Ban Ki-moon, the UN secretary-general, said 16 of the world body's personnel were confirmed dead late Wednesday, and 100 to 150 were still unaccounted for.
"It is our estimate that around hundreds of people were still working inside the building," he said.
"Therefore it will be in the range of 100 to 150 that I'm quite concerned about."
He said 11 Brazilian peacekeepers and five international police officers - three from Jordan and one each from Chad and Argentina - were killed in the "horrendous" quake, and that Hedi Annabi, the mission chief, was among those missing.
John Holmes, the UN humanitarian chief, could not confirm if Annabi was dead and said that the UN was "trying not to be distracted by our own casualties".
"Our first priority is to help the Haitian people, get them out and save them while they're still alive," he told Al Jazeera.
Colonel Romeo Brawner, the spokesman for the Philippines' army, which contributes with troops to the UN force, said four Filipino peacekeepers were trapped under the rubble.
He said the rest of the Philippines' 157 troops were busy in search and rescue operations.
"Most of our soldiers are now very busy digging through the rubble, with their bare hands," he told Al Jazeera.
"Unfortunately, we're not equipped with specialised equipments to go into the rubble, not even listening devices, but they're doing their best to help out in the rescue operations."
The UN's mission in Haiti, spread across the country, includes about 7,000 peacekeeping troops, 2,000 international police, 490 international civilian staffers, 1,200 local civilian staffers and 200 volunteers.
The force was brought in after the president was ousted in a bloody 2004 rebellion following decades of violence and poverty in the nation.
One of the areas where the mission has been operating is the slum of Cite Soleil. Several peacekeepers and Haitians have been killed in clashes between UN troops and armed gangs, and residents have blamed the UN forces for the death of numerous unarmed bystanders.
Eugenia Charles, the executive director of the Fondasyon Mapou, a Haitian human rights organisation, told Al Jazeera that many Haitians are suspicious towards the UN mission.
"We can go back and see the number of people they have killed in Cite Soleil ... There is still a lot of resentment.
"[The UN has] been in Haiti since 2004. They've had the opportunity to help in rebuilding infrastructure but many of them spend their time going to beaches.
"They have seen what's needed in terms of roads, in terms of schools, hospitals. They could have done more."