A strong aftershock rocked Haiti on Wednesday morning just as much-needed medical aid via a U.S. Navy ship reached the earthquake-ravaged nation.

Patients at a hospital near Haiti's airport in Port-au-Prince immediately started praying as the ground shook like a ship rocking back and forth. They asked for forgiveness and protection, a nurse said.

At least one injury was reported in the moments after the 6.1-magnitude aftershock, which struck at 6:03 a.m. ET. There were no immediate reports of deaths or damage.

The aftershock was the strongest to hit Haiti since January 12's 7.0-magnitude earthquake, the U.S. Geological Survey said.

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The Haitian government said it already has recovered 72,000 bodies, but the Pan American Health Organization, which is coordinating the health-sector response, has offered a preliminary estimate of 200,000 dead.

The aftershock rattled people struggling to recover from last week's quake that walloped the impoverished country.

Such a strong tremor can pose significant danger in a nation where damaged buildings are teetering precariously.

The 7.0 earthquake was 32 times stronger in terms of magnitude -- or energy released -- than the 6.1 temblor, said Carrieann Bedwell, a geophysicist with the USGS. That difference is what people feel on the ground, she said.

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Early Wednesday, 3 million Haitians -- a third of the population -- were still in need of food, water, shelter and medical assistance, the United Nations estimated.

There are scenes of "madness" in some parts of the capital, CNN's Ivan Watson reported from near the city's port.

People had gathered, desperate to catch a boat to another part of the country, though there was no certainty that such vessels were coming.

Families were rowing out to sea in small, overloaded rowboats -- carrying up to 25 people -- to be first in line if a ferry did arrive, he said.

The situation was "a disaster waiting to happen," Watson added, saying there was no leadership and no information whatsoever.

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But relief workers were making strides in the arduous effort of getting aid to victims, officials said Wednesday.

"Every day we reach out further," said Lt. Gen. P.K. Keen, deputy commander of the U.S. military's Southern Command. "We are moving in the right direction."

The U.S. military has delivered 200,000 bottles of water and more than 600,000 rations to Haitians, Keen said.

John Holmes, U.N. emergency relief coordinator, agreed that the situation is slowly improving.

"We are beginning to turn the corner. We are making progress," said Holmes. "But it's very frustrating that it takes so long to get as many supplies, doctors and hospitals that are needed."

The aid effort has frustrated some, with a few organizations charging that bottlenecks at the Port-au-Prince airport and mismanagement in other areas has hampered efforts to get help to 2 million residents in the capital.

But a state-of-the-art hospital aboard a U.S. naval ship reached Haiti on Wednesday.

The U.S. Navy ship Comfort arrived Wednesday in the flattened capital. U.S. helicopters will ferry patients aboard, bringing relief to overloaded hospitals and clinics.

Two severely injured Haitians already have been transported to the hospital ship as it sailed toward Haiti, the U.S. military said.

The patients -- a 6-year-old boy with a crushed pelvis and 20-year-old man with a broken skull and possibly fractured cervical vertebrae -- had been treated initially on the USS Carl Vinson, a U.S. aircraft carrier docked off the Haitian capital.

The Comfort is carrying nearly 550 doctors, nurses, corpsmen, technicians and support staff, who will be joined by 350 other medical staffers once the ship reaches Haiti, according to the Southern Command. The ship will have six operating rooms available and can house up to 1,000 patients.

More than a week after the devastating earthquake, the injured still streamed in to hospitals that were struggling to get back into working order.

Surgeries resumed Tuesday at University Hospital, the country's largest, said Dr. Jon Andrus, deputy director of the Pan American Health Organization.

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At the general hospital in Port-au-Prince, doctors were working under stressful conditions in facilities away from the main building, which was deemed unsafe.

"We have run out of IVs and IV needles and IV fluids," said Dr. Mark Hyman of the medical relief group Partners in Health. "We've run out of surgical supplies. We have to wash with vodka, and we have to operate with hacksaws because we don't have enough operating tools."

And the death toll is expected to climb.

Haitian Prime Minister Jean-Max Bellerive told CNN's Christiane Amanpour that at least 72,000 bodies had been recovered, not including the unknown number buried by families or collected by the U.N. peacekeeping mission in Haiti.

It was unclear how many of the dead had been identified before burial and how many of those burials occurred in mass graves.

At least 28 Americans were confirmed dead, the U.S. State Department said Tuesday.

But miraculous rescues were still taking place as people were being pulled out alive even after a week under rubble.

On Tuesday night, New York City Fire Department and Police Department rescuers pulled two children alive from the ruins of a two-story Port-au-Prince building. The boy, 8, and girl, 10, were taken to an Israeli tent hospital.

Earlier, rescuers pulled a survivor from the rubble near Haiti's national cathedral. Rescue crews said two other survivors might be under the same pile, though hopes faded in the evening.

In all, international rescue teams of about 1,700 people have rescued 121 people, the United Nations said.

Outside Haiti, people have contributed more than $220 million to major U.S. relief groups, according to The Chronicle of Philanthropy, a newspaper covering nonprofit organizations.

But the world's generosity continued to overwhelm the ability of the airport in Port-au-Prince. The result: Some badly needed aid was left sitting on the tarmac.

Some flights were diverted to Santo Domingo, Dominican Republic, causing congestion there, too, Andrus said.

Haitian President Rene Preval applauded the progress aid workers have made in restoring electricity and communication, clearing roads, erecting shelters, distributing food and re-establishing hospitals.

Preval credited the international community.

"Without their help, it would be impossible for us to cope with the situation," he said in an interview with CNN's Amanpour.

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