Mexico yesterday asked the United States to extradite a senior Drug Enforcement Administration agent and a key DEA informant to face charges of illegally abducting a defendant indicted in the 1985 torture-murder of DEA agent Enrique "Kiki" Camarena.

The request for the extradition of DEA agent Hector Berrellez and informant Antonio Garate Bustamante caught U.S. officials off guard and threatened new tensions in U.S.-Mexican relations, sources said.

Justice Department officials also expressed concerns that the Mexican demand could interfere with the prosecution of Humberto Alvarez Machain, the gynecologist who was abducted last April in Guadalajara and brought to the United States to face charges that he helped Mexican drug traffickers torture Camarena.

"On its face, this is not helpful and it is not a cooperative step for law enforcement," said one Justice Department official. A State Department official confirmed that the extradition requests had been received and were being studied but declined further comment.

Berrellez is the DEA agent who heads Operation Leyenda, a nine-member unit based in Los Angeles that has been conducting a five-year search for Camarena's murderers. At a May 25 court hearing in Los Angeles, Berrellez testified that, with the approval of senior DEA officials, he authorized former Mexican drug agent Garate to arrange the abduction of Alvarez Machain and pay a $50,000 reward plus expenses to his abductors.

Alvarez Machain was kidnapped in his office on April 2, blindfolded and flown to El Paso, where he was arrested by DEA agents the following day. The abduction, carried out by a group of seven Mexicans, including some police officers, prompted Mexican officials to denounce the United States for violating its sovereignty and threatened to cut off anti-drug cooperation.

Justice Department officials believed the controversy had died down in the past month, pointing to progress in working out new cooperative arrangements between DEA and the Mexican federal police. But earlier this week, officials learned that the Mexican attorney general's office, which had vowed to conduct a thorough investigation of the affair, had filed criminal charges against Berrellez, Garate and other Mexicans implicated in Alvarez Machain's abduction. "Apparently, it is a kidnapping case," said an official.

The development comes while a jury in Los Angeles continues to deliberate the fate of four defendants, including the brother-in-law of former Mexican president Luis Echeverria, who are charged with participating in Camarena's murder in Mexico in February 1985. The trial of Alvarez Machain has been postponed pending a ruling from U.S. District Judge Edward Rafeedie on the legality of his arrest.