FREETOWN, SIERRA LEONE, AUG. 7 -- The leaders of five West African nations agreed today to send troops to Liberia to impose a cease-fire in an eight-month-old civil war that has claimed at least 5,000 lives and spurred countless tribal atrocities.

The leaders, meeting in the Gambian capital of Banjul, said a five-nation military force led by a Ghanaian commander and Guinean deputy commander would go to Liberia to enforce a truce and monitor a transition to an interim government. The other nations involved are Nigeria, Sierra Leone and Gambia.

The leaders did not say when the effort would begin or how many troops would be involved. They did say it would cost about $50 million and that they would seek U.S. assistance.

Meanwhile, more foreign nationals, including at least four Americans, were evacuated by U.S. military helicopters from the besieged Liberian capital of Monrovia as a rebel leader seeking to provoke greater foreign intervention to end the conflict reportedly threatened to begin targeting U.S. Marines for attack.

The rebel leader, Prince Johnson, said the 235 Marines assigned to protect U.S. property and rescue foreign nationals would be subject to attack by rebels to heighten international pressure to intervene, according to reports from Monrovia.

The Marines were flown into Monrovia Sunday from warships off the Liberian coast to evacuate Americans and other foreign civilians following a previous threat by Johnson to begin taking hostages.

On Monday, Johnson's troops reportedly seized 15 foreigners, including one American and three Britons, from a Monrovia hotel and took them away as hostages. He later displayed eight of them to reporters and said they would not go free until a peace-keeping force comes to Liberia.

In Washington, State Department spokesman Margaret Tutwiler said a representative of Johnson told U.S. officials in Monrovia today that "the people held will not be harmed and that they are all well."

Nevertheless, Marine helicopters took out 19 people from Monrovia on Monday and another eight today, she said. Tutwiler responded to Prince Johnson, saying the "Marines are in Monrovia to protect the embassy and the Americans there. They will do that with force if necessary. They are not in Monrovia to intervene in the fighting or to take sides."

Johnson once served as an officer in the National Patriotic Front, the rebel movement led by Charles Taylor that launched the offensive against the government of Liberian President Samuel Doe last December. Johnson and several hundred followers bolted the movement several weeks ago and vowed to prevent Taylor from taking power in Liberia. Both rebel factions are seeking to end the 10-year rule of Doe, who remains holed up in his fortified mansion in Monrovia with about 500 troops.

A spokesman for Doe interviewed by the British Broadcasting Corp. tonight said he welcomed the decision by the West African states to intervene. Sierra Leone, into which more than 50,000 refugees have streamed since the fighting began, could serve as a staging ground for such a joint military effort.

The Americans and other foreign nationals who arrived by helicopter in this dilapidated capital today were the latest of more than 90 civilians who have been evacuated from Monrovia since the Marines landed Sunday. During interviews today, several described scenes of anarchy, horror and death.

"The smell is terrible. So many bodies are just rotting in the streets," said a British businessman who declined to be further identified, citing the security of friends and co-workers in Monrovia. "I have seen them kill for a dollar. When Johnson's men began to loot my warehouse I knew it was time to go."

A British national who said he had lived in a neighborhood of southeastern Monrovia that Johnson's troops controlled described the rebel faction leader as "a very frightened young man who is desperate for help."

"He lived in my house for two days. We drank my whiskey," the businessman said. "But I think this strain is getting too much for him. He is cracking."

The Britons said Johnson became disturbed when foreigners living in parts of the city he controlled began to leave last week. "He was angry because he said he had been good to all of them and that they shouldn't abandon him now," one Briton said.