Down in the hull, everything is ready to go. There are tractors and trucks and three huge landing craft. There's water purifying equipment, plastic tarps and wood beams for building temporary shelters.

And there are more than 1,300 Marines ready to take it all ashore and get to work.

But — even for a strictly humanitarian mission — in the political minefield of southern Asia, getting American boots on the ground is a delicate concept.

"We the Marines are biting our nails," said Colonel Thomas Greenwood, commanding officer of the 15th Marine Expeditionary Unit. "We want to get in and start doing our job."

The first helicopter flights off this Navy amphibious assault vessel began relief operations on Tuesday, flying to the city of Medan on Indonesia's (search) tsunami-struck island of Sumatra, where more than 100,000 people are feared dead and millions more are homeless after the catastrophic Dec. 26 earthquake and tsunami.

Plans to put a Marine expeditionary unit ashore on Sri Lanka (search) with heavy equipment, however, have been put on hold. After being informed that Colombo was scaling down its request for help, this ship and the USS Duluth canceled plans to spearhead relief efforts off Sri Lanka's coast and have instead joined the aircraft carrier USS Abraham Lincoln (search) and its battle group off Sumatra.

The USS Mount Rushmore, carrying a smaller contingent of Marines, will likely travel on to Sri Lanka alone. It was expected to cross the Indian Ocean by the weekend. An advance party of seven Marines arrived in the southern town of Galle Tuesday.

Though no firm plans had been set, due to the uncertainty of the situation, the Marines had hoped to put more than 1,000 troops ashore in Sri Lanka to help clear roads and build shelters for refugees.

The Bonhomme Richard, carrying more than 1,300 Marines, has three landing craft aboard that float on air cushions and are capable of putting the troops ashore by the hundreds on almost any kind of beach.

All are fully loaded and ready to go.

But for the time being, that capability will not be used. Instead, the ship's helicopters will continue ferrying supplies to and from the regional airports where they have been piling up, and taking them out to the more remote places where they are needed.

Though that mission is crucial, part of the reason behind the recalibration is political.

Sri Lanka has long been embroiled in a civil war with the Tamil Tiger rebel group, and the area of operations in Sumatra's Aceh province is also highly sensitive. Due to a long-standing insurgency, Aceh had been restricted to foreigners.

Jakarta was quick to open Aceh because it needed the help, but the image of large numbers of Marines pouring ashore would be politically sensitive to the predominantly Muslim nation.

Greenwood stressed that the Marines were aware of the concerns.

"We don't want to offend anybody's sensitivities, he said. "The alleviation of suffering and the loss of human lives should trump politics. We want to be helpful without being bothersome."

Washington has been keen to display the humanitarian capabilities of its military.

All told, about 20 military ships and more than 10,000 Marines and sailors have been mobilized for the relief operation, which is the largest the U.S. military has conducted in Asia since the Vietnam War ended in 1975 and possibly its biggest humanitarian mission ever. Air Force C-130 cargo planes have been flying sorties out of the Vietnam-era bomber base in Utapao, southern Thailand.

With Sri Lanka no longer on its itinerary, the Bonhomme Richard was expected to take its position off southern Sumatra, while the Lincoln battle group would remain in the more heavily populated north.

The extent of death and damage in south Sumatra is not yet well known, and one of the Marines' first priorities was to conduct reconnaissance missions to determine what areas needed the most help.

The military's helicopter operations have been key to easing aid bottlenecks and getting supplies out to the harder-to-access areas. But the Marines are also hoping to put troops on the ground to provide badly needed manpower for clearing roads and airfields and for building shelters for refugees.