The UN security council voted unanimously yesterday to approve an additional force of 3,500 police and soldiers for earthquake-devastated Haiti, amid rising concern over outbreaks of looting by desperate survivors and the re-emergence of notorious gang leaders who escaped when the country's prisons collapsed.

The extra security forces will bolster the UN stabilisation force of some 7,000 troops who are already in the country.

The vote came as relief efforts turned from the search for survivors buried under the rubble to the delivery of much needed food, water, shelter and medical assistance to some 250,000 Haitians in most need, many of them living on the streets ,who have seen little or no aid a week after the catastrophe.

The reinforcement, requested by the UN secretary general, Ban Ki-moon, who visited Haiti on Sunday, is intended to bring under control outbreaks of looting and violence that have slowed distribution of supplies to the survivors of the earthquake that killed an estimated 200,000 people.

The police chief in Port-au-Prince, Mario Andersol, said he could muster only 2,000 officers in the capital, down from 4,500 before the quake, and they were "not trained to deal with this kind of situation". On Monday the World Food Programme only managed to feed half the 100,000 people it planned to reach because security forces were not available to escort its trucks.

The vote at the UN came as a further 2,000 US marines landed in Haiti, bringing the numbers who have deployed for the disaster to 11,000, including scores of US paratroopers who landed at the damaged presidential palace.

Amid continuing concerns over a lack of co-ordination in the emergency efforts, which has seen only a trickle of promised aid reach hundreds of thousands of Haitians in desperate need, American commanders also announced that they hoped to open two more runways at Port-au-Prince's airport within the next few days.

The US military, which began limited airdrops of food and water, including some 14,000 ready-to-eat meals and 15,000 litres of water, said today it was now considering airdrops across Haiti. Previously it had ruled out airdrops because of fears they might spark riots.

The U-turn has been forced by an acute shortage of fuel that has been hampering efforts to distribute aid that has already arrived in the country. Many motorists say they have tried for days to fill empty tanks or jerrycans at service stations that jacked up prices and rationed gasoline sales. Most of the people on the wrecked streets of Port-au-Prince are on foot because their motorcycles or cars have run dry.

"Fuel has become a critical issue in terms of being able to get vehicles with water and food out to people," a spokeswoman for the World Food Programme, Emilia Casella, said in Geneva.

The latest increase in US and peacekeeping forces came as paratroopers from the 82nd Airborne Division were landed by helicopter at the partly collapsed presidential palace. They then marched to Port-au-Prince's nearby general hospital, apparently to protect international medical teams who are racing against time to tend seriously injured quake victims. While army paratroopers took charge of key sites in the capital, US marines were also landing south of the city to link up with UN peacekeepers and prepare to land a larger flow of troops and equipment. The increased US deployment, while applauded by some Haitians, was criticised by others who see it as an "occupation".

Medical teams pouring into Port-au-Prince to set up mobile hospitals said today they were still overwhelmed by the casualties and warned of the immediate threats of tetanus and gangrene as well as the spread of measles, meningitis and other infections. The World Health Organisation said today at least 13 hospitals were now working in or around Port-au-Prince.

The UN agency was bringing in emergency medical supplies today to treat 120,000 people over the next month, WHO spokesman Paul Garwood told reporters in Geneva. "We are not past the emergency phase yet, but we are starting to look at the long-term," said Margaret Aguirre, of the International Medical Corps, whose staff had helped with 150 amputations so far.

Taiwan and Venezuela - Haiti's main bilateral creditors - today came under intensifying pressure to cancel the impoverished Caribbean country's debts. Haiti owes $167m (£102m) to Venezuela and $91m to Taiwan.

The Paris Club - an informal group of 19 creditor governments from industrialised countries that includes Britain - agreed last July to cancel their claims on Haiti, totalling $214m.Britain was one of the first members of the Paris Club to cancel all dets owed to it by Haiti.