Haitians sought comfort in their faith Sunday, flocking to pray in church ruins as rescue teams raced against time to pull out any final survivors five days after a devastating earthquake.

UN chief Ban Ki-moon flew in to assess what he called the "most serious humanitarian disaster in decades," while security degenerated in the capital with police killing a man as they fired on looters ransacking a market.

The leading US general on the ground warned that 200,000 might be a reasonable "start point" for the eventual toll, but said it was still too early to predict a figure that might never be accurately known.

"Clearly, this is a disaster of epic proportions, and we've got a lot of work ahead of us," said Lieutenant-General Ken Keen, who is running the vast US military relief operation in the stricken Caribbean nation.

Haitian officials and the Red Cross have said around 50,000 people perished in the quake, but this figure could be conservative as tens of thousands of rotting bodies have already been buried, many in mass graves.

After hours of painstaking digging through the ruins, a team from Florida unearthed a seven-year-old girl, a man aged 34 and a 50-year-old woman in the ruins of a supermarket as dawn broke in the capital Port-au-Prince.

A Dane was later pulled alive from the rubble of the UN mission headquarters without a scratch on him, but rescuers knew the likelihood of finding more survivors was waning with every passing hour.

Hundreds of rioters ransacked Hyppolite market in the heart of the devastated capital as survivors besieged hospitals and make-shift field clinics, some carrying the injured on their backs or on carts.

Police reinforcements descended on the market armed with shotguns and assault rifles and one rioter, a man in his 30s, was fatally shot in the head, an AFP photographer said.

The church bells lay eerily silent Sunday over the ruined capital, but the faithful still gathered in great numbers to pray for solace in the darkest hour of this deeply religious nation.

"I want to send a message of hope because God is still with us even in the depths of this tragedy, and life is not over," said father Henry Marie Landasse as he held Mass in the ruins of the main cathedral.

Prayers among the rubble for Haiti's faithful

But with vital supplies of water and food still struggling to reach some of those most in need, many Haitians were close to despair.

"Life is really hard, we have nothing," said 40-year-old Jean Osee, camped out with his entire family in front of the presidential palace.

"I don't have much strength to feed him, I can't look after him properly," his daughter Louisoguine said, cradling her curly-haired baby.

Little relief for Haitian family in palace slum

Lieutenant-General Keen vowed to redouble efforts after 70,000 bottles of water and 130,000 food rations were distributed on Saturday, while President Barack Obama signed an order mobilizing military reserves, particularly medical staff to work on hospital ships.

Water purification units that can process 100,000 liters (26,417 gallons) of clean water per day were being rushed to the scene as the US worked to open badly damaged ports needed to deliver vital fuel and supplies.

The US military has been relying mainly on helicopters deployed from the USS Carl Vinson aircraft carrier.

The Medecins Sans Frontiers (Doctors Without Borders) aid group said that when it opened an emergency hospital at Carrefour, a poor district near Leogane on Saturday, crowds arrived almost immediately.

"Patients arrived on handcarts or on men's backs," said MSF emergency coordinator Hans van Dillen.

"There are other hospitals in the area, but they are already unable to cope with the number of injured and have limited resources of personnel and medicines and equipment."

Haitian radio key link for survivors

MSF said their doctors and surgeons had been working around the clock, amputating limbs and performing caesarian sections on pregnant women.

The United Nations has estimated that three million people were affected, and 300,000 left homeless. Some 40 tent cities have sprung up in Port-au-Prince, according to the Red Cross.

Ban says Haiti worst disaster in decades

UN Office for the Coordination of Humanitarian Affairs spokeswoman Elisabeth Byrs said 12 people had been pulled out alive from the debris on Saturday, taking the total to more than 70 since the teams started working. AFP knew of just the four on Sunday.

"We don't give up hope to find more survivors," stressed Byrs. "The morale of the rescue team is very high despite the hardship."

But Rami Peltz, a rescuer with an Israeli team, said: "Today is the last day that I think we will be able to find survivors, mainly because of dehydration."