Liberia's warring sides signed a cease-fire agreement yesterday after 11 months of brutal civil and tribal warfare, but the accord appeared to undermine a new interim government installed one week ago by a West African peace-keeping force.

There were no immediate details of the cease-fire, signed in Bamako, Mali, by rebel leader Charles Taylor, whose forces started the fighting last Christmas Eve, and representatives of a breakaway rebel army led by Prince Johnson and of the remaining government forces of slain president Samuel K. Doe.

The signing of the agreement after the first face-to-face talks among all parties to Liberia's conflict was a small triumph of normalcy in a country where just two months ago, Doe's pet lions reportedly were eating starving people who had taken refuge in his presidential palace.

The cease-fire marked the emergence of the Economic Community of West African States as a regional power and peacemaker. The signing capped a 48-hour meeting between the Liberian groups and leaders of 13 ECOWAS nations, which sent a peace-keeping force to Liberia in August.

In Washington, the State Department acknowledged the role of ECOWAS in bringing Liberia to a point that the United States found impossible to attain during the last 11 months of fighting.

"We . . . commend the statesmenlike efforts of all the parties involved, including the chiefs of state of the ECOWAS nations in their efforts to settle this conflict in their region," a State Department spokeswoman said. The United States helped found Liberia in 1822 and had been heavily influential in Liberian affairs, both politically and financially, until the civil war broke out.

Nigerian President Ibrahim Babangida, who attended the signing, said the predominantly Nigerian peace-keeping force would remain in Liberia to oversee the truce. He also said Nigeria, an oil-producing country that stands to prosper from the price rise brought on by the Persian Gulf crisis, will send aid money to Liberia.

"We have a commitment to assist the country in its rehabilitation efforts when peace returns, and I am hoping that will be in the next six months," Babangida said, according to the Associated Press.

The Nigerian air force began bombing Taylor's strongholds last month, and to avoid the air strikes Taylor fled to neighboring Ivory Coast, according to an interview earlier this month with Edward B. Kesselly, who had been designated the new interim government's defense minister.

On Nov. 14, ECOWAS planes bombed the port of Buchanan, held by Taylor's forces, which retaliated with artillery barrages against Monrovia's port, controlled by the West Africans. The effect of these battles was to cut relief supplies to Monrovia, the capital, where 50 to 60 people reportedly have been dying each day of starvation and disease.

The fighting has killed more than 10,000 Liberians. By some estimates, about half of the country's 2.3 million people have fled to neighboring countries.

Yesterday, Gambian President Dawda Jawara, the ECOWAS chairman, who also signed the cease-fire, hailed the agreement as "a breakthrough," according to Reuter, but shortly after the signing, Taylor made clear that the accord did not provide for a political settlement of the conflict.

"The document that we signed states very clearly that no group in Liberia is recognized as an interim government," Taylor said.

Also attending the talks was Amos Sawyer, who was sworn in as Liberia's interim president a week ago in Monrovia under the auspices and protection of the West African task force. According to Kesselly and S. Byron Tarr, the new finance minister, the Sawyer government was comprised of nine intellectuals, all of whom had been in exile under Doe and -- by design -- represented all of Liberia's ethnic groups and regions.

Sawyer left Bamako yesterday without making a statement, and a communique issued at the end of the summit referred to a "future interim government," according to Reuter. The warring sides agreed to start talks with other Liberian groups on a new interim government to prepare the country for elections.