NAIROBI, Kenya (AP) _ Tigre rebels seeking to topple the government claimed Thursday to have captured a town 65 miles west of Ethiopia's capital, Addis Ababa.

There was no immediate government comment on the claim, but Western diplomats in Addis Ababa say the insurgents pose the greatest threat ever to the Marxist government of President Mengistu Haile Mariam, who overthrew Emperor Haile Selassie in 1974.

The U.S. State Department said Thursday it is ordering all embassy dependents and non-essential personnel out of Ethiopia because of the rebel advances.

All private Americans are being urged to leave the country as soon as possible, department spokesman Richard Boucher told reporters in Washington.

There are about 600 Americans in Ethiopia on private business or working with aid organizations trying to cope with a famine that threatens at least 5 million people.

Forces of the Ethiopian People's Revolutionary Democratic Front won the desert town of Ambo on Wednesday, a spokesman for the Front said.

Many foreign diplomats consider the control of Ambo critical. The town is near one of the country's main ammunition factories.

Fighting was continuing with remaining government troops who were dug in about 10 miles north of the town on the main road to the capital, Front spokesman Asefa Mamo said in a telephone interview from its London office.

He described the government soldiers as elite forces that were being reinforced by crack troops from Asmara, in the country's north and Dessye, to the east.

Mamo said rebels eventually planned to march on the capital but said that was the ''final goal ... not the immediate goal.''

The Tigreans have been fighting since 1975 seeking Mengistu's ouster and greater autonomy for their home province, Tigre.

Last month, many embassies, including that of the United States, advised their dependents and non-essential staff to leave the country on a voluntary basis following dramatic rebel gains.

However, it was not immediately known whether any other foreign missions also planned to upgrade their advisories to ordered departures and further reduce the number of their personnel.

All sources in Addis Ababa spoke to The Associated Press in telephone interviews.

The Front is a combined force of the Tigrean People's Liberation Front and the much smaller Ethiopian People's Democratic Movement, another group of fighters from the north-central province of Tigre.

Since launching an offensive Feb. 23, the insurgents have made striking gains, winning control of the northwestern provinces of Gondar and Gojjam, both important grain-growing regions.

They also took significant portions of the western province of Welega, home to Ambo, and portions of the central and eastern provinces of Shewa and Welo.

The Tigre rebels won control of their home province of Tigre in late 1989.

Combined with territory held by a separate insurgency, the secessionist Eritrean People's Liberation Front, Ethiopian rebels now control Ethiopia's northern third.

The Eritreans reportedly have been fighting alongside the Tigreans in some battles and have been coordinating their separate advances with the EPRDF offensive.

They currently threaten Assab, the last port under government control, according to diplomatic sources in Addis Ababa.

The government has heavily defended the port, which handles about 70 percent of the nation's foreign trade, has the nation's sole fuel refinery and is key to the delivery of relief food destined for many famine victims.