UN chief Ban Ki-moon said Tuesday's 7.0 magnitude quake, which raised fears that as many as 100,000 people may have been killed, was "a tragedy for Haiti and... for the United Nations."
Haitian Prime Minister Jean-Max Bellerive told CNN that the death toll in Haiti's worst quake in more than 160 years, could be "well over 100,000," although he added: "I hope that is not true."
"This is going to be one of the highest in terms of loss of life in recent years, as far as we can tell," US Secretary of State Hillary Clinton however said as she announced she was cutting short her trip to Asia to head straight back to Washington.
She gave no figure but compared the Haiti quake to the Indian Ocean tsunami that killed more than 220,000 people five years ago.
The UN mission in Haiti (MINUSTAH) was hit particularly hard, with its main headquarters building in Port-au-Prince flattened and dozens of staff missing, including its civilian head, Hedi Annabi of Tunisia, and his deputy.
Haitian President Rene Preval insisted that Annabi, who was in the building along with dozens of his staff when the quake struck Tuesday, was killed.
But like other UN officials, UN chief Ban Ki-moon said he could not confirm Preval's claim.
The respected Tunisian official, who served as deputy head of UN peacekeeping operations from 1997 to 2007, was having talks with a visiting Chinese delegation in the five-story concrete building known as the Christopher Hotel, UN chief Ban Ki-moon said.
Citing preliminary figures, Ban said late Wednesday that a total of 16 UN personnel were confirmed dead in the Haitian capital: three police officers from Jordan, one from Argentina, one from Chad and 11 Brazilian peacekeepers.
Eight Chinese members of the peacekeeping mission were also buried in building collapses, according to China's State Council. It did not say whether they were believed to be alive or dead.
Susana Malcorra, head of the UN department of Field Support, earlier put the total number of UN personnel still unaccounted "in the range of 150."
On a more positive note, Michele Montas, a former spokeswoman for the UN chief who was visiting her family in Haiti when the powerful earthquake struck Port-au-Prince, was accounted for, according to Farhan Haq, a UN spokesman.
"The secretary general is very relieved" to hear that Montas, a former award-winning Haitian journalist who retired in early December after a distinguished UN career, is safe, Haq said.
The tragedy is the worst disaster to hit the world body since the August 19, 2003 suicide attack on UN offices in Baghdad, in which 22 people, including UN special envoy Sergio Vieira de Mello, were killed.
In December 2007, two suicide blasts targeting UN offices in Algiers killed at least 41 people, including 18 UN staffers, three of them foreign nationals.
Ban sent Edmond Mulet, a top UN peacekeeping official and Annabi's predecessor, to Haiti to take charge of the decapitated MINUSTAH in an interim capacity.
And the UN boss added that he himself was prepared to visit Haiti "as soon as practically possible."
Ban said the UN system was mobilizing an emergency response team to coordinate relief aid that was expected to arrive in Port-au-Prince soon.
"We are extremely concerned about the humanitarian impact" in the city of 2.8 million people, where the national palace, the main prison, hotels, hospitals, and schools sustained extensive damage," said John Holmes, the head of the UN Office for the Coordination of Humanitarian Affairs (OCHA).
"Basic services such as water, electricity, have collapsed almost entirely," Ban said.
Holmes said a Chinese search and rescue team had arrived in Port-au-Prince, where the airport was operating despite damage sustained by the control tower.
Two US teams were believed to have landed as well, he added.
UN officials said they would launch a major flash appeal for funds in the next two to three days and had already released ten million dollars in emergency aid.
MINUSTAH, which comprises 7,060 troops and 2,091 police, has been deployed since mid-2004 to help stabilize the impoverished Caribbean island-nation of nine million people.
Keep up with the latest business news from the region with the Emirates Business 24|7 daily newsletter. To subscribe to the newsletter, please click here