MONROVIA, LIBERIA, JUNE 4 -- Four U.S. warships carrying 2,000 Marines arrived off Liberia today as rebels advanced on the airport outside Monrovia. The U.S. Embassy said the ships would help evacuate Americans from Liberia if necessary.

President Samuel K. Doe begged "all peace-loving nations," including the United States, "to come to the aid of the Liberian people" as rebels captured a key army checkpoint near the international airport outside Monrovia and advanced on the city.

"It is the wishes of the people of this country that America can do something to stop the bloodshed," he said in a telephone interview from Liberia with Cable News Network.

The U.S. State Department has accused rebel leader Charles Taylor of receiving some support from Libya, but it also has said Marines will not intervene in support of Doe's government.

More than 50 U.S. Embassy workers, missionaries and their families and hundreds of Liberians crowded another airport inside the capital amid fears there would be a bloodbath if the rebels took the city.

State Department spokesman Margaret Tutwiler said in Washington, "There has been fighting within the vicinity of Roberts Field International Airport within the last 24 hours. We understand the rebels are within a few miles of Roberts Field and have taken the town of Owens Grove.''

British Airways manager David Ranger said rebels seized the army checkpoint seven miles east of the international airport, which is 35 miles southeast of the capital.

No defense lines were visible in the capital or on the road to the airport.

Doe's military commander, Lt. Gen. Henry Dubar, appeared to be counting on the Marines to step in and save the capital.

"The armed forces of Liberia would not consider a Marine landing an invasion," he said. "We are very happy about them coming. It is long overdue. The rebels are Libyan-backed so we felt the United States should come to our aid immediately. Just the presence of the Marines here will scare the rebels away."

The State Department last week ordered all nonessential U.S. government personnel and their families to leave immediately. About 75 Americans remained at the embassy, said spokesman David Krekke.

At Spriggs Payne Airfield, about five miles from the city center, about 50 Americans, 20 South Koreans and a few British and Canadians left aboard a chartered Boeing 737 to Freetown, capital of Sierra Leone.

The rebels are mostly members of the Gio and Mano tribes and invaded the country of 2.5 million people from the neighboring Ivory Coast about five months ago.

Doe has ordered his troops into their barracks because of charges they were assaulting and killing members of tribal groups who have supported the rebels. Doe has been holed up in his presidential mansion in the capital. He is protected by about 1,000 Israeli-trained troops.

In the city, people of the Mandingo and Krahn tribes loyal to Doe piled their belongings into taxis, trucks and buses, fleeing reprisals by rebels threatening to attack the capital.

"If they come, they'll massacre us. So we're going," said a father of four children as he stacked a mattress onto the back of a truck. His family and two others were heading for Sierra Leone.

Rebels have killed Krahn and Mandingo civilians in retaliation for government troop atrocities against Gio and Mano civilians. International human rights organizations say the soldiers have killed hundreds of civilians and the rebels scores.

{Meanwhile, diplomats in Liberia said the United States appeared to be seeking a negotiated settlement, the Reuter reported. Government sources and diplomats said a settlement would probably involve Doe's departure. The Liberian Council of Churches has been in contact with the rebels, they said.}