BUCHANAN, LIBERIA, JUNE 5 -- Liberian rebels fighting to overthrow the government of President Samuel Doe said today they had attacked a U.S.-managed rubber plantation and positions near Robertsfield International Airport, both about 30 miles southeast of the capital, Monrovia.

Diplomats in Monrovia quoted by news agencies reported that the rebels of Charles Taylor's National Patriotic Front had captured the Bridgestone-Firestone Inc. plantation, but government officials said their forces continued to hold Liberia's principal airport, next to the plantation.

Taylor's forces continued to assemble in this port, which was largely deserted, with many shops and homes looted. Twenty bodies, most of them in civilian clothes, could be seen lying in the streets.

Some people here told an Agence France-Presse correspondent that they saw rebels summarily execute hundreds of people from rival tribes, although that account could not be confirmed independently. The sources cited by AFP said the victims were primarily members of Doe's tribe, the Krahns, and an allied tribe, the Mandingos. Most of Taylor's rebels are members of the Gio and Mano tribes.

Western journalists with Taylor's forces have observed numerous cases of indiscipline that indicate that Taylor has had trouble building a command structure through which to control rebel soldiers.

Last week, Taylor assured a Western journalist that his forces did not commit indiscriminate killings, and that they had been told to imprison, rather than kill, government troops.

Within 20 minutes, however, members of Taylor's forces nearby roughed up a man in civilian clothes whom they suspected of being a Krahn tribesman. The man protested that he was a member of another tribe, the Krus.

After holding the man for two hours, rebels led him a short distance away. Three shots exploded and the man could be seen lying on the ground, a patch of blood on his back. The man's executioner walked back, chanting: "For the revolution!"

Rebel sources said the gunman was later arrested and is being held.

Taylor, 42, once was the head of the government purchasing agency, but fled Liberia in 1983 when Doe accused him of embezzling $1 million. He reentered the country from the Ivory Coast last December with a small group of soldiers and gained fighters as government troops were accused of atrocities against civilians amid the fighting.

The Liberian army chief of staff, Gen. Henry Dubar, told journalists in Monrovia today that a former U.S. Marine, identified as Elmer Johnson, had been killed in battle. Dubar described Johnson as "the brains behind" Taylor's military campaign.

Rebel sources here confirmed that Johnson, a military adviser to Taylor, was missing after an ambush three days ago. They said Johnson was a Liberian national.

{A Marine Corps spokesman in Washington said there was not sufficient information to determine if Johnson had been a Marine. Foreign citizens and dual nationals are permitted to serve in the U.S. armed forces, he said.}

According to local residents, Taylor's forces at first captured Buchanan without a fight, occupying the town on May 19, as its 2,000 government soldiers fled. Several days later, however, 500 government troops returned and battled the rebels, leading most of Buchanan's residents to flee. It is impossible to tell how many people were killed in the fighting here; some residents said as many as 1,000 died.

News agencies reported from Monrovia:

Residents at the Bridgestone-Firestone plantation said rebels fired mortars and automatic weapons as dozens crossed the Farmington River -- the last natural barrier before Monrovia -- and entered the plantation. A U.S. Embassy official said several American managers were at the site, but they did "not appear to be in imminent danger" and no immediate evacuation was planned.

Fearful residents fled the capital, many searching for alternative routes of escape following the cancellation of international flights from the Robertsfield airport. Some government soldiers abandoned key posts, fearing rebel attacks.

Doe remained at his presidential mansion, guarded by about 1,000 Israeli-trained troops. He continued last-ditch efforts to end the civil war, meeting church leaders who have offered to mediate.