WASHINGTON — One of the consequences of a Delta Air Lines merger with Northwest Airlines Corp. might be a dramatic increase in the amount of aircraft maintenance work going overseas, Teamsters President James Hoffa said Monday.

"Northwest does an extensive amount of outsourcing overseas," Hoffa said in an interview. The prospect of sending more jobs to Asia "is a key concern that should be addressed" in any regulatory review of a possible merger, he said, adding that it carries implications for safety.

Delta said the assertion that maintenance work will be sent overseas is unfounded because the Atlanta-based carrier profits from doing maintenance work for other carriers.

"Delta senior leadership has made very clear its commitment to growing its in-house maintenance business by focusing on high-skill, high-value maintenance work," Delta spokeswoman Chris Kelly said. "Last year, Delta TechOps brought in more than $377 million in revenue, and 2008 is looking to be even better."

The Teamsters union, which does not represent workers at either Delta or Northwest, says major U.S. carriers now spend nearly two-thirds of their maintenance dollars on contract repair stations, which include facilities in China, El Salvador, Mexico and the Philippines.

Hoffa spoke at a Teamster-sponsored conference on aircraft maintenance outsourcing. He and other speakers said this increasing reliance on foreign labor represents a threat to U.S. travelers' safety because other countries do not have the same drug and alcohol testing standards for workers, and do not conduct thorough background checks to weed out criminals and terrorists.

"Aircraft mechanics should all be held to a single standard whether they repair airplanes in Beijing or San Francisco," Hoffa said. "Unfortunately, the Federal Aviation Administration doesn't have the staff or funding" to enforce U.S. standards when work is being done overseas.

The Federal Aviation Administration certifies 4,187 domestic and 709 foreign repair stations. FAA spokeswoman Alison Duquette did not have statistics to show the growth overseas, but agreed "the amount of work outsourced definitely has been increasing."

The FAA says its inspectors do travel to foreign facilities. "The standards for domestic and foreign repair stations are the same," Duquette said. "The FAA has not seen a problem with outsourced maintenance."

According to Transportation Department data collected by Massachusetts Institute of Technology, Northwest spends more on outsourcing than Delta.

Northwest spent $647 million on "outsourced flight equipment maintenance expenses" in 2006, the most recent full-year data available. That compared with $261 million for outsourcing just two years earlier.

At Delta, spending on outsourcing rose to $467 million in 2006, up from $271 million two years earlier.

William Swelbar, an airline researcher at MIT, said he doubts the quality of maintenance work done overseas is inferior because airlines have too much to lose by taking risks. "I do not believe they would do anything to marginalize safety," Swelbar said.

But Paul Bradley, a United Airlines mechanic who works at Dulles International Airport outside Washington, said he believes the maintenance work being done in Beijing for United is subpar.

"Once the planes come back [from Beijing], we spend two or three weeks trying to fix the things they screwed up overseas," said Bradley, who attended the conference. He has been working to persuade United's mechanics to switch their representation from the Aircraft Mechanics Fraternal Association to the Teamsters.

The Teamsters, along with conference co-sponsor the Business Travel Coalition, urged Congress to consider legislation to increase the frequency and depth of FAA inspections at domestic and foreign repair stations, to require foreign contractors to do criminal background checks and drug and alcohol screenings, and to set uniform standards for all repair stations.

Speaking to the conference in a videotaped statement, House Transportation Committee Chairman James Oberstar (D-Minn.) said Congress will "hold [airlines'] feet to the fire" to make sure they are not compromising safety.

Oberstar has said repeatedly that he would oppose a Delta-Northwest merger, in part because it would eliminate many U.S. jobs. Similarly, he said Monday he opposes aircraft maintenance work moving overseas because "these are good-paying jobs."