The Pentagon plans to send 200 more U.S. troops ashore in Liberia (search) to help Nigerian peacekeepers (search) get food and other emergency supplies flowing and to provide a land-based "quick-reaction force" in case the peacekeepers get in trouble, officials said Wednesday.

The plan reflects a somewhat more extensive U.S. military involvement than Pentagon officials had suggested a day earlier, although it is smaller than Liberian officials have requested.

Pentagon officials stressed that the 200 U.S. troops -- which are in addition to roughly 100 already there as liaison with the Nigerians and as security for the U.S. Embassy -- will not directly do peacekeeping duty. Rather they will be acting in support by, for example, providing engineering work at Monrovia's port to help get ship-borne humanitarian supplies in.

The plan for bringing in the extra 200 U.S. troops is contingent on rebels withdrawing from the Monrovia port on Thursday as they promised after Charles Taylor flew into exile on Monday.

"The objective is to let the Nigerian forces continue with stabilizing key areas of the city" that are needed to provide humanitarian relief, said Lawrence Di Rita, the acting assistant secretary of defense for public affairs.

Air Force Lt. Gen. Norton Schwartz, director of operations for the Joint Chiefs of Staff (search), told a Pentagon news conference that at least 150 U.S. Marines would go ashore from Navy ships off the Liberian coast as a backup combat force that could quickly come to the aid of African peacekeepers.

The Marines won't see combat except in an emergency, and they would not be equipped with armored vehicles, Schwartz said. He would not say where in Liberia they will be based.

Other officials said the quick-reaction force is expected to go ashore as soon as an additional battalion of Nigerian peacekeepers arrives in Monrovia, the Liberian capital, near the end of this week.

There already are about 750 Nigerian peacekeepers on the scene, and the expectation is that once another group of similar size joins them they will have sufficient strength to stabilize the situation. At that point -- perhaps as soon as next week -- the U.S. quick-reaction force would move back aboard their Navy ships off the coast, the other officials said.

That would have the troops in Liberia for only several days, although no specific time limit has been set.

Di Rita said the intention is to withdraw the land-based reaction force as soon as the second Nigerian battalion arrives, but he added, "that's subject to change, and as it develops they'll reassess that."

In addition to the 150 or more Marines in the quick-reaction force, smaller numbers of U.S. troops will perform other missions:

-- About a dozen Navy SEALs, the Sea-Air-Land forces that are the Navy's special operations troops, will search the waters of the Monrovia port to clear any obstacles to the arrival of ships carrying food aid. A preliminary search of those waters began Wednesday, and Schwartz said a more extensive search will be done when the Nigerian peacekeepers begin moving to the port.

-- U.S. Harrier fighter jets or Huey or Cobra helicopters will provide what Schwartz described as "aerial escort" as the Nigerian troops head from the airport to the port at Monrovia.

-- Small numbers of U.S. troops will be "embedded" with the Nigerians as they head to the port, to act as liaison with the commander of U.S. forces in the area, Army Lt. Gen. Thomas Turner.

-- Military engineers will assess the port facilities to determine what improvements are required.

The three Navy ships off the Liberian coast are carrying the 26th Marine Expeditionary Unit, which has about 2,300 Marines aboard with a wide variety of capabilities, including ground combat power as well as forces trained to conduct hostage rescue and noncombatant evacuations.

Schwartz said there was no plan to bring any of the three ships into Monrovia's port.