At least 482,000 people have quit Haiti's capital Port-au-Prince since the Jan. 12 earthquake, while international relief work is making progress, said the United Nations on Wednesday.

They have gone to their relatives or friends for shelter in nearby towns which are not affected by the earthquake, said the United Nations Office for Humanitarian Affairs.

Haitian authorities have helped 235,000 people leave Port-au-Prince, most of whom were taken to nearby villages with temporary shelter.

According to the UN, food was running out in several towns due to an increased influx of people, which is also straining the treatment capacity of the medical centers there.

Prime Minister Jean Max Bellerive said Wednesday that more than 200,000 people were killed in the devastating earthquake, adding that 300,000 others were injured, including 4,000 amputees.

Non-governmental organizations have warned that an estimated 3,000 people who lost their limbs would face a tough time returning to normal life.

Doctors now operating on Haitians with the most severe injuries are trying their best to save their patients' limbs, with amputation as the last-ditch option.

However, several sources said during days after the quake, too many amputation operations were performed inappropriately by inexperienced doctors.

Head of the United Nations Children's Fund (UNICEF), Ann Veneman, is scheduled to arrive in Haiti on Thursday to help launch a massive immunization campaign, which will vaccinate 2.5 million Haitians.

"The time is critical in this kind of thing because if you start to get an outbreak then it can spread very quickly in the unhygienic conditions that exist in Haiti," UNICEF spokesperson Kate Donovan told Xinhua Wednesday.

"Children are 100 percent vulnerable because many children are malnourished so their natural resistance to any disease is really reduced."

The immunization campaign, which targets measles, diphtheria and tetanus, initially aims to vaccinate 530,00 children under the age of seven. However, UNICEF hopes to eventually reach 2.5 million adults throughout Haiti, said Donovan, who preferred not to give a timeline due to the sheer difficulty of such an operation.

"It's a massive vaccination campaign that's going to start in stages," she said. "Initially, we'll start in Port-au-Prince and then we'll go out to (other) affected areas."

UNICEF and the World Health Organization (WHO) will set up immunization centers and make an attempt to find those most vulnerable, said Donavan.

UNICEF has also warned that incidents of child trafficking are often reported. In last week alone, 10 American Baptist missionaries were arrested when they were caught trying to cross into the Dominican Republic from Haiti with a busload of 33 children they said were orphaned after the quake. However, Haitian police said some of the children have living parents.

UN Secretary-General Ban Ki-moon said Wednesday that the relief operations remain complex, but significant progress is being made in relief operations.

"The situation is largely calm," he said. "Food distribution is growing smoother by the day, and we have now reached about 1 million people. Banks, markets and schools are beginning to re-open."

The UN chief has asked former U.S. President Bill Clinton, already deeply involved in Haiti before last month's devastating earthquake, to assume a leadership role in coordinating international aid efforts, from emergency response to reconstruction.

"The secretary-general said he could not think of a better suited leader than President Clinton to do this," UN spokesman Martin Nesirky told a news briefing. "The aim is to provide strategic guidance to the UN involvement at international level."

Clinton will work together with UN Development Program (UNDP) Administrator Helen Clark and Ban's Special Representative Edmond Mulet.

On food distribution, the World Food Program (WFP) has scaled up its operation. The agency has now reached almost 1 million people since the quake struck, with more than 300,000 gaining access through a coupon-based distribution system in the last 72 hours.

The UN Stabilization Mission in Haiti reported Wednesday that the security situation remains calm, despite isolated incidents in the past few days.