The 6.1-magnitude aftershock was about 6.2 miles deep, with an epicenter about 35 miles (60 kilometers) west-southwest of the capital of Port-au-Prince, the U.S. Geological Survey said.

It rattled people struggling to recover from the devastating 7.0-magnitude earthquake that walloped the impoverished country January 12, killing at least 72,000 people.

Such a strong tremor can pose significant danger in a nation where damaged buildings are teetering precariously. The aftershock was the strongest to hit Haiti since last week's original quake, the USGS said.

At least one injury was reported in the moments after the aftershock, which struck at 6:03 a.m. ET.

The aftershock jolted Haiti as much-needed medical reinforcement approached offshore in the form of a state-of-the-art hospital aboard a U.S. naval ship.

The USNS Comfort is to arrive midmorning 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, according to the U.S. Department of Defense.

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 U.S. Southern Command. The ship will have six operating rooms available and can house up to 1,000 patients.

The United States has been conducting some medical operations aboard the USS Carl Vinson, docked off Haiti's coast.

More than a week after the devastating earthquake, efforts to get hospitals back into working shape were seeing some results, but the injured were still streaming in.

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

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

"We have run out of IVs and IV needles and IV fluids," said Dr. Mark Hyman of the medical relief organization 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."

The military is going to help with organization and supplies, Hyman said. "They're going to help us get electricity, they're going to help us get food, they're going to help us get tents, they're going to help us get all the operating supplies in," he said.

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

And the death toll is expected to climb.

Authorities have buried about one-third of the estimated final toll. Prime Minister Jean-Max Bellerive told CNN's Christiane Amanpour that at least 72,000 bodies had been recovered, not including the unknown number of bodies 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.

"We know that bodies have been buried; we feel inappropriately," Andrus said. He cited lack of refrigeration as a complicating factor and made an urgent appeal for blood donors because storage was not possible.

"Despite all our efforts, situations, circumstances are such that we are disappointed in many cases on how this has been managed, beyond everybody's control," he said.

Pan American Health Organization, which is coordinating the health-sector response, offered a preliminary estimate of 200,000 dead.

At least 28 of them were Americans, 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 8-year-old boy and 10-year-old girl 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.

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