Haiti is in badly need of doctors and medical supplies, the top United Nations humanitarian coordinator said on Thursday.

"The local medical infrastructure is both badly damaged and overwhelmed by the number of injuries," said John Holmes. "It's a top priority to get more doctors in there, more medical teams, field hospitals and more medical supplies to make sure we can tackle that problem."

After that, the basic needs are water, food and emergency shelters, added Holmes.

As rescue teams from several countries, including China and the United States, sweep Haiti's capital, Port-au-Prince, in search of people who might be buried alive from Tuesday's earthquake, the UN family has been leading relief efforts.

Aid has been pouring in from around the world but an overwhelmed airport, among other problems with infrastructure, has delayed distribution, Holmes said.

"The blockage is in the arrival of aid," he told reporters. "There is a a large number of planes trying to get in there."

The international airport was reopened on Wednesday after the U. S. military was able to take charge of a temporary control tower. The original one collapsed during the earthquake.

Haiti's main seaport was not as lucky. "Certainly damaged," the port is surrounded by debris in the water which could make access difficult, said Holmes, noting that shipping in aid might be cheaper than air cargo in the long-term.

Holmes praised the international community's quick response to such a massive and devastating tragedy.

The United Nations is expected on Friday to launch a flash appeal for international assistance in the amount of hundreds of millions of U.S. dollars. However, like any flash appeal, said Holmes, the sum asked for will be rough and will most likely be revised in three or four weeks.