Washington ? The United States may send small numbers of additional Marines ashore from ships off Liberia’s coast if U.S. commanders decide they are needed to start humanitarian aid flowing, a senior defense official said Tuesday.

The official, who discussed the situation on condition of anonymity, said no decisions have been made to expand the U.S. military presence in Liberia beyond the nearly 100 there now. But he said discussions were under way with West African peacekeeping commanders to determine what is needed.

In Liberia on Tuesday, the leading rebel movement agreed to pull back from Monrovia by noon Thursday and surrender control to peacekeepers, allowing food to flow in the war-torn country.

U.S. Ambassador John Blaney called the agreement “extremely important for the delivery of humanitarian relief.” The deal obliges the West African peacekeeping force to speed up its deployment.

The senior U.S. defense official cited three examples of potential additional U.S. military involvement:

• Troops to act as liaison between West African peacekeeping battalions and Army Maj. Gen. Thomas R. Turner II, commander of the U.S. military task force assigned to the Liberia mission. Turner was in Monrovia, the Liberian capital, on Tuesday to meet with West African military commanders and others.
• Engineering experts to assess Liberian port facilities that are to be used to receive humanitarian aid.
• Medical specialists with capabilities not presently available in Monrovia.

In each of those three cases, a small military security force probably would be included, the official said.

The additional troops likely would be Marines from the three-ship Amphibious Ready Group that is afloat off Monrovia’s shore. There are about 2,300 Marines and 2,500 sailors aboard the ships.

Tuesday’s accord with the rebels came as a second rebel group launched a push on Monrovia, sending refugees fleeing vicious new assaults a day after warlord-turned-president Charles Taylor resigned and went into exile. Witnesses reported machete-wielding fighters were attacking indiscriminately near the airport.