The Red Cross estimates that between 45,000 and 50,000 people might have died in the 7.0-magnitude earthquake that devastated the Haitian capital, Port-au-Prince.

The Haitian Red Cross based the casualty figure on reports from a network of volunteers across the city, said Jean-Luc Martinage, a spokesman for the International Federation of Red Cross and Red Crescent Societies.

"We consider this as an estimate," Martinage said.

Emergency aid from around the world began arriving in ravaged Haiti on Thursday as the frantic search for survivors continued.

However, relief groups said the destruction from the deadly earthquake was making it difficult to get supplies to people.

Roselyn Joseph mourns over the body of her daughter, Emanuela Aminise, 14, in Port-au-Prince, Haiti, on Thursday. (Carl Juste/The Miami Herald/Associated Press)

"It's chaos," UN humanitarian spokeswoman Elisabeth Byrs told The Associated Press. "It's a logistical nightmare."

President Barack Obama and the U.S. moved to take charge in Haiti on Thursday, dispatching thousands of troops along with tons of aid to try to keep order as well as rescue the suffering in a country that's dysfunctional in the best of times.

He pledged an initial $100 million with the likelihood of more later.

Planes carrying teams from China, France and Spain also landed at Toussaint L'Ouverture International Airport in Port-au-Prince with searchers and tonnes of food, medicine and other supplies. A Canadian CC-177 military plane with emergency supplies also arrived around midday Thursday.

CBC News reporter Sasa Petricic arrived on that plane with the advance element of Canada's Disaster Assistance Response Team. He said the jet had to circle the Port-au-Prince airport for about three hours before it could land, because the tarmac was too crowded with aircraft.

An Air China plane carrying a Chinese search-and-rescue team, medics and aid landed at the capital city's airport on Thursday, led by more than 50 people in orange jumpsuits accompanied by trained dogs.

"Most of the members are very experienced," Liu Xiangyang, deputy chief of the National Earthquake Disaster Emergency Rescue Team, told the official Xinhua News Agency before its departure, referring to the May 2008 quake in southwestern China that left almost 90,000 people dead or missing.

Three French planes touched down Thursday to transport about 60 injured people to hospitals in the Caribbean islands of Martinique and Guadeloupe.

While the airport in the capital was damaged and the control tower collapsed in the quake, the runways are clear for landing.

But the World Food Program said the airport is straining to handle dozens of incoming flights of supplies and rescuers. It also remains difficult to travel from the airport to the city as debris from collapsed buildings is littering the streets.

As well, the organization said damage to the port in Port-au-Prince is preventing ship deliveries to the quake-struck region.

John Holmes, the UN's relief co-ordinator, praised the international community for its response, but he acknowledged that the people in desperate need in Haiti may feel that help isn't coming fast enough.

"Inevitably the reality is that, however fast we try to move, it will always be too slow for those people who are on the ground who are waiting impatiently for help, and that's something which is very frustrating, particularly for them, but, of course, it's very frustrating for us too," he said.

"If we could snap our fingers and make these things arrive, we would do that, but that's not possible," Holmes told reporters in New York City.

Canada has promised humanitarian aid to the island. Two ships — HMCS Athabaskan and HMCS Halifax — departed from Halifax Thursday afternoon loaded with supplies and equipment, including a helicopter, first-aid kits and 500 Canadian soldiers.

The International Red Cross has estimated three million people — a third of the population — may need emergency relief that includes shelter, food and clean water.

A map shows the epicentre of the quake. (CBC)

"We'll be using whatever roads are passable to get aid to Port-au-Prince, and if possible we'll bring helicopters in," said Emilia Casella, a spokeswoman for the United Nations food agency in Geneva.

Survivors used sledgehammers and their bare hands to try to find victims in the rubble. Survivors were being transported by pickup trucks, wheelbarrows and doors converted into makeshift stretchers.

Thousands of Haitians spent another night outside as survivors set up camps amid piles of salvaged goods, including food scavenged from the rubble. Many won't return to their homes, fearing aftershocks that have continued to hit the country will knock down the already weakened structures.

Thousands of homes, including the National Palace, have been destroyed or damaged.

UN casualties

UN officials in Port-au-Prince confirmed that 19 UN peacekeepers, four international police officers and 13 UN staff members are dead. About 100 UN workers are trapped in the rubble of the UN headquarters that collapsed in the quake, while another 50 UN staff are unaccounted for elsewhere.

David Wimhurst, a senior UN official in Haiti, said more bodies are expected to be found in the coming days.

United Nations Secretary General Ban Ki-moon earlier said there was a "small miracle" as one Estonian security guard, Tarmo Joveer, was pulled alive from the destroyed UN headquarters.

Wimhurst said another seven rescues were made at the headquarters building.

UN mission head Hedi Annabi of Tunisia and his chief deputy, Luis Carlos da Costa, are among the missing.

Canadian Kim Bolduc, the UN Development Program co-ordinator in Haiti, is now heading the mission. Leadership will be taken over by Edmond Mulet, the UN assistant secretary general for peacekeeping operations, when he arrives.

Speaking to reporters via video conference, Bolduc said it was "a real challenge" for the UN with their top management missing.

Four Canadians — a Montreal couple, an Ontario nurse and an RCMP officer from New Brunswick — have been confirmed among the dead.

Calls from victims seeking help from emergency services weren't getting through because systems that connect different phone networks were not working, said officials from a telecommunications provider in Haiti.

About 3,000 police and international peacekeepers cleared debris, directed traffic and maintained security in the capital.

But law enforcement was stretched thin even before the quake and would be ill-equipped to deal with major unrest. The UN's 9,000-member peacekeeping force sent patrols across the capital's streets while securing the airport, port and main buildings.

Looting began immediately after the quake, with people seen carrying food from collapsed buildings. Inmates were reported to have escaped from the damaged main prison in Port-au-Prince, said the UN's Byrs.

The quake struck at 4:53 p.m., centred 16 kilometres west of Port-au-Prince at a depth of eight kilometres, the U.S. Geological Survey said. USGS geophysicist Kristin Marano called it the strongest earthquake since 1770 in what is now Haiti, on the island of Hispaniola.

With files from The Associated Press