The US military is ramping up its mission in quake-hit Haiti, with 20,000 US troops expected to operate on ground and offshore by Sunday, the US commander overseeing the region said.

General Douglas Fraser, the head of US Southern Command (SOUTHCOM), said Thursday that over 2,676 US troops were currently operating on the ground in Haiti to boost the still struggling aid effort in the aftermath of a killer quake.

He said that number was expected to swell to 4,600 by the weekend, and that another 10,445 were currently afloat aboard vessels offshore.

Over 4,000 other soldiers and Marines from the Nassau Amphibious Ready Group and the 24th Marine Expeditionary Unit left North Carolina late Wednesday, scrapping scheduled deployments to Europe and the Middle East.

They were expected to arrive on Sunday, Fraser said, bringing the total US military contingent to nearly 20,000 as survivors still scramble for food, water and even the most basic of medical supplies in the aftermath of the catastrophic January 12 temblor.

"We're still looking where the specific requirements are that we need them," Fraser told reporters at the Pentagon via live video feed from Miami.

"Again, the security environment is calm, remains stable, but we're getting a demand to escort humanitarian assistance to various parts, and so we were uncertain of what the demand was going to be, and so I asked for that capability just so I was prepared to meet the demand."

The general said the influx of additional forces will "give us an increased capacity to move humanitarian supplies throughout the country."

Despite the additional stress on an already thinly stretched military, the head of US Central Command said the Haiti relief effort would have "no impact whatsoever" on plans to ship out some 30,000 additional troops to Afghanistan.

"We're also obviously watching very carefully as we are providing resources to the government of Haiti," said General David Petraeus, who oversees the wars in Iraq and Afghanistan.

Three amphibious ships -- the USS Nassau, the USS Mesa Verde and the USS Ashland -- will support the Haiti deployment, along with a helicopter squadron, a tilt-rotor squadron and medical personnel and facilities.

The Nassau has one of the largest ship-borne hospitals after the USNS Mercy and USNS Comfort hospital ships. Staffed with a crew of 850, the Comfort has already treated more than 230 emergency patients and will eventually provide around 1,000 hospital beds, according to the White House.

Over 7,000 patients have been treated from five medical teams of the US Health and Human Services Department and one US-funded International Medical Surgical Team.

SOUTHCOM has contracted for the purchase of 50,000 handheld radios -- now the main means for most Haitians to reach out to lost family members and learn about new developments. Nearly 44,000 have already arrived in Port-au-Prince and the remainder are due by Monday.

The United States has distributed over 600,000 water bottles and 400,000 meals in recent days, while hundreds of thousands more gallons (liters) of water were being produced using water treatment units.

Total food and non-food assistance for Haiti from the US Agency for International Development has reached nearly 165 million dollars.

Some 20 US Navy and Coast Guard ships, 63 helicopters and 204 vehicles were supporting the effort.

The airport in Port-au-Prince is now supporting about 120 to 140 flights a day but the massive influx of international aid means there is a waiting list of over 1,400 flights waiting to land, according to Fraser.

Another three airports have been opened to facilitate the transfer of much-needed supplies.

The Haitian capital's badly damaged port, meanwhile, was beginning to receive ships and was about 30 percent operational, the White House said.