WASHINGTON (AP) _ The Bush administration today deplored the killing of two nuns in Nicaragua but said the leftist government in Managua has failed to back up its charge that the attack was carried out by the U.S.-backed Contra rebels.

White House spokesman Marlin Fitzwater said it was not known who carried out the ambush Monday night on a car carrying the Roman Catholic church workers on a highway in a remote northern section of the country.

Two nuns - one American, the other Nicaraguan - were killed, and a third was injured, as was an American-born Nicaraguan bishop.

''We deplore in the strongest terms this outrageous attack on religious workers and extend our deepest sympathy to the families of the victims. This is a tragic incident, one that further underscores the need to bring peace, stability and democracy to Nicaragua,'' Fitzwater told reporters.

''The U.S. Embassy in Nicaragua has been in contact with local church officials to obtain information, including the Sisters of Saint Agnes and the Capuchin Fathers,'' he said.

''We have no information on who was reponsible for this attack, and we note that the church officials themselves have no information, either,'' Fitzwater continued.

''The Nicaraguan government has provided no information to substantiate its charge that the Contras launched the attack, and we hope the Sandinistas do not seek to obscure this tragedy by engaging in a propaganda battle.''

Fitzwater said the Contras had condemned the attack and its military commander said the attack occurred outside of the Contras' usual operating area.

Six Jesuit priests were slain in El Salvador recently a few days after leftist rebels launched an offensive against the government of Alberto Cristiani. Church groups have charged government security forces may have carried out those killings.

The Nicaraguan government recently broke off a cease-fire with the rebels. Elections are scheduled in the country next month, the first since the Sandinistas toppled right-wing dictator Anastasio Samoza and came to power in 1979.