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* Traumatized survivors begging for food and water

* U.S. official says security situation "pretty good"

* Washington says flow of aid should accelerate

* Troops, doctors, planes full of food head for Haiti (Recasts, adds details, U.S. defence secretary)

By Catherine Bremer and Andrew Cawthorne

PORT-AU-PRINCE, Jan 15 (Reuters) - Thousands of people left hurt or homeless in Haiti's earthquake begged for food, water and medical assistance on Friday as the world rushed to deliver aid to survivors before their despair turned to anger.

Tens of thousands are feared dead from Tuesday's massive quake. The Pan American Health Organization estimated the death toll could be 50,000 to 100,000, higher than previous figures from the Haitian Red Cross, which saw deaths at up to 50,000.

Citizens in the wrecked coastal capital Port-au-Prince spent a third night sleeping out in the open on sidewalks and streets strewn with rubble and scattered decomposing bodies, as aftershocks rippled through the hilly neighborhoods.

Governments across the world were pouring relief supplies and medical teams into the quake-hit Caribbean state -- already the poorest in the Western Hemisphere. But huge logistical hurdles and the sheer scale of the destruction meant aid was still not reaching hundreds of thousands of victims.

"We have lost everything. We are waiting for death. We have nothing to eat, nowhere to live. We have had no help. No one has come to see us," said quake victim Andres Rosario, speaking at an improvised camp set up by survivors at a rubbish dump in Port-au-Prince.

"No one is helping us. Please bring us water or people will die soon," said another resident Renelde Lamarque, who has opened his home yard to about 500 quake victims in the devastated Fort National neighborhood.

Raggedly-dressed survivors held out their arms to foreign reporters in the streets, begging for food and water.

Amid fears that local anger and frustration over delays in receiving help could explode into violence, U.S. Defense Secretary Robert Gates said that aside from some scavenging for supplies and minor looting the security situation on the ground in Haiti remained "pretty good."

"The key is to get the food and the water in there as quickly as possible so that people don't in their desperation turn to violence or lead to the security situation deteriorating," Gates told reporters in Washington. The United States is leading a massive international relief effort.

"BY THE GRACE OF GOD"

Police have all but vanished from the streets, and although some Brazilian U.N. peacekeepers were patrolling, there have been reports of sporadic scavenging and some looting.

At one destroyed supermarket scores of people swarmed over the rubble to try to reach the food underneath. Just outside Cite Soleil slum, desperate people crowded around a burst water pipe jostling to drink from the pipe or fill up buckets.

Some survivors, angry over the delay in getting aid, build roadblocks with corpses on Thursday in one part of the city.

Relief workers said some aid was trickling through to people but in haphazard fashion. "Some aid is slowly getting through, but not to many people," said Margaret Aguirre, a senior official with International Medical Corps.

The United States said the arrival of its nuclear-powered aircraft carrier USS Carl Vinson with 19 helicopters on Friday would open a second significant channel to deliver help.

"Up until now we've been delivering assistance through a garden hose but now we are expanding that," U.S. State Department spokesman P.J. Crowley said. But Port-au-Prince's airport had limited capacity and the port remained unusable.

At the airport, now under the control of the U.S. military, planes were arriving every 20 minutes, from small to large.

But in streets strewn with rubble, garbage and rotting bodies, most Haitians said they had still received nothing.

"I haven't eaten since the day before yesterday," said Bertilie Francis, 43, who was with her three children.

"We are here by the Grace of God, nobody else," she said.

Health experts say that while dead bodies smell unpleasant, in cases where people have been killed in traumatic accidents and not by contagious diseases such as cholera there is little health risk from even large numbers of decomposing corpses.

Local radio stations were broadcasting messages for people to put their dead out in the street to be picked up by trucks and taken to a mass grave.

On a barren area in the hillsides outside the city, a Reuters reporter found nine recently dug mass graves for victims -- two were already covered up, six had bodies piled inside and a seventh was empty. President Rene Preval has said at least 7,000 quake victims have already been buried.

Aguirre said aid agencies were discussing setting up a central refugee camp to try to group a multitude of victims' settlements springing up all over Port-au-Prince.

NEED FOR COORDINATION

"The key is the coordination ... We want to avoid people just running round doing their own thing," she said.

In a sign that international relief efforts cut across ideological differences, communist-led Cuba agreed to let the U.S. military use restricted Cuban air space for medical evacuation flights carrying Haitian earthquake victims, sharply reducing the flight time to Miami, a U.S. official said.

United Nations disaster experts said at least 10 percent of housing in the Haitian capital was destroyed, making about 300,000 homeless, but in some areas 50 percent of buildings were destroyed or badly damaged.

U.N. aid agencies were to launch an emergency appeal for approximately $550 million on Friday to help survivors.

The U.N. peacekeeping mission in Haiti, which has lost at least 36 of its personnel in the quake, was trying to provide some basic coordination from an office near the airport.

U.N. Secretary-General Ban Ki-moon said he planned to go to Haiti "very soon"

In the capital overnight, an eerie chorus of hymns, prayers, groans and wails of mourning, mixed with the barking of terrified dogs, echoed over the hilly neighborhoods.

Bodies lay all around the hilly city, and people covered their noses with cloth to block the stench of death.

U.S. President Barack Obama pledged an initial $100 million for Haiti quake relief and enlisted former U.S. presidents Bill Clinton and George W. Bush to help raise more, vowing to the Haitian people: "You will not be forsaken." [ID:nN14198913]

The United States was sending 3,500 soldiers, 300 medical personnel, several ships and 2,200 Marines to Haiti.

Nations around the world pitched in to send rescue teams with search dogs and heavy equipment, helicopters, tents, water purification units, food, doctors and telecoms teams. But aid distribution was hampered because roads were blocked by rubble and smashed cars and normal communications were cut off. (Additional reporting by Tom Brown, Kena Betancur and Carlos Barria in Port-au-Prince, Stephanie Nebehay in Geneva and Steve Holland in Washington; writing by Anthony Boadle and Pascal Fletcher; editing by David Storey)