PORT-AU-PRINCE - U.S. Black Hawk helicopters swooped down on Haiti's wrecked presidential palace to deploy troops and supplies on Tuesday as a huge relief operation to help earthquake survivors gained momentum.

The airborne troops in combat gear moved to secure Port-au-Prince's nearby General Hospital, where staff have been overwhelmed by huge numbers of seriously injured patients.

Their dramatic deployment to help speed up logjammed aid efforts brought crowds of quake survivors camped out in the park opposite the palace rushing to its iron railings to gawk and beg for handouts of food.

It was the most visible and potentially sensitive deployment so far by the U.S. military, which is spearheading international efforts to assist millions of Haitians left injured or homeless by the devastating earthquake a week ago.

Venezuela's socialist President Hugo Chavez, a fiery critic of what he calls U.S. imperialism, has accused Washington of occupying Haiti under the pretext of an aid operation.

The commander of the U.S. troops in Haiti, Lieutenant General Ken Keen, said their primary purpose was humanitarian assistance and providing food and water to Haitians, but that there was also a security element to the operation.

Security goes hand-in-hand with our mission, he told CNN at the hospital protected by his men.

Watching the soldiers, quake survivor Gille Frantz said: We know the world wants to help us, but it has been eight days now and I have not seen any food or water for my family.

In a bid to accelerate the arrival of aid and stem looting and violence, the U.N. Security Council unanimously agreed to temporarily add 2,000 U.N. troops and 1,500 police to the 9,000-member peacekeeping mission in Haiti.

More than 11,000 U.S. military personnel are on the ground, on ships offshore or en route, and Haitian President Rene Preval has said U.S. troops will help U.N. peacekeepers bring order to the capital's increasingly lawless streets.

Hundreds of looters have swarmed over wrecked stores in downtown Port-au-Prince since the quake struck, seizing goods and fighting among themselves.

LOOTERS OUTNUMBER POLICE

Haiti's Police Chief Mario Andresol said his depleted force needs the help of U.N. peacekeepers to maintain law and order, noting that 4,000 criminals escaped from damaged prisons.

Yesterday, downtown the looters simply outnumbered my guys. They could not control them, Andresol told Reuters.

It will be very difficult to get the bad guys back into jail, Andresol said. We were in control of security before the earthquake, but now we do not have the conditions to fix the situation. That is why we need outside help.

U.N. officials said the security situation had not hampered distribution of food rations to 270,000 Haitians so far and that hundreds of thousands more would benefit in coming days.

The situation is tense but calm. Of course there are lootings because the population is on edge, Elisabeth Byrs, spokeswoman of the U.N. Office for the Coordination of Humanitarian Affairs, said in Geneva.

Haitian officials say the death toll from the magnitude 7 earthquake that destroyed much of the capital on January 12 was likely to be between 100,000 and 200,000. So far, they have buried 75,000 bodies in mass graves.

Some 52 rescue teams from around the world are in a race against time to find people still alive under the rubble of collapsed buildings. They have saved around 90 people so far, including two on Monday.

THREAT OF DISEASE

Medical teams pouring into Port-au-Prince to set up mobile hospitals said they were 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 Organization, or WHO, said at least 13 hospitals were working in or around Port-au-Prince and that it was bringing in emergency supplies to treat 120,000 people over the next month.

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.

There is a risk of cholera and tetanus, and a huge need for mobile medical units, she told Reuters in Port-au-Prince.

AIDS, tuberculosis and malaria are rampant in Haiti, many children are malnourished and hygiene was a challenge even before the quake.

Under the protection of U.S. troops, food and water and other supplies have begun arriving more regularly at the congested U.S.-run airfield in Port-au-Prince.

U.S. military officers hope to reopen the shattered seaport in two or three days, but are relying for now on airdrops of food and water to those waiting in makeshift refugee camps.

The U.N. World Food Program said 270,000 people had been given food assistance by Monday night and that it aimed to send out 10 million ready-to-eat rations over the next week, enough to feed half a million people three times a day for a week.

While a few street markets began selling fruit, vegetables, charcoal, chicken and pork, tens of thousands of survivors across the city are still clamoring for help.

Fuel prices have doubled, and there were long queues outside gas stations, where cars, motorbikes and people with jerrycans have lined up. Haitian police stood guard at some.

Former U.S. President Bill Clinton, the U.N. special envoy to Haiti, did his bit for the aid effort on Monday by unloading bottles of water from a plane after landing in Port-au-Prince.

It's astonishing what the Haitians have been able to accomplish, performing surgeries at night ... with no anesthesia, using vodka to sterilize equipment, he said after touring a hospital where supplies were very tight.

World leaders have promised massive amounts of assistance to rebuild Haiti and Preval appealed to donors to focus not just on immediate aid for Haitians but also on long-term development of the poorest nation in the Western Hemisphere.

President Leonel Fernandez of neighboring Dominican Republic proposed the creation of a $2 billion-a-year fund to finance Haiti's recovery over five years.

The United States agreed to take in Haitian orphans who are being adopted by U.S. citizens and are legally confirmed as eligible for adoption abroad by Haiti.

U.S. President Barack Obama's handling of the Haitian crisis so far won the approval of 80 percent of Americans polled by CBS News and may have helped a rise in his overall job approval rating to 50 percent from an all-time low of 46 percent last week, CBS said.

(Additional reporting by Carlos Barria, Andrew Cawthorne, Catherine Bremer in Port-au-Prince, Stephanie Nebehay in Geneva, writing by Anthony Boadle, editing by Kieran Murray)