WASHINGTON (AP) _ Cross-border smuggling usually means to U.S. officials the flow of illegal drugs from Mexico. Mexican officials worry about a radically different kind of illicit traffic going the other way: guns.

They're expected to complain about Mexico-bound American firearms to President Clinton and his delegation of Cabinet members during their trip to Mexico City this week for official talks.

American officials admit that illicit trafficking exists and is difficult to stop, given the massive number of vehicles that transit the border. To inspect them all is unrealistic, they say.

But as bilateral issues go, gun smuggling has lacked the high profile of such as drugs, migration and trade, but it's a subject that increasingly worries officials in Mexico.

``Our preliminary information is that most of the illegal gun trafficking from the U.S. goes to narcotics traffickers,'' said Mexico's ambassador to Washington, Jesus Silva Herzog.

He suspects some weaponry sent to Mexico originates in gun shops along the U.S. side of the 2,000-mile border. He said they number in the thousands.

Mexican officials determined that a gun shop in Texas was the source of the weapon used in the 1994 assassination of the ruling party's presidential candidate, Luis Donaldo Colosio. And a southern California gun dealer was found to have supplied 170 guns taken from a Tijuana-based drug gang.

U.S. officials say many disreputable border gun shops have been shut down in recent years.

Another complaint of Mexican authorities are lightly regulated U.S. ``gun shows,'' which they describe as open-air markets for buying and selling weapons.

Mexican anxiety over gun smuggling was reinforced in March by a U.S. Customs Service raid on a California warehouse that yielded two truckloads of illegal arms. They entered the country through Long Beach port after an excursion that began in Vietnam.

The arsenal comprised disassembled grenade launchers and fully automatic M-2 carbines. U.S. investigators said it was earmarked for Mexico.

Despite such apparent lapses, American officials question the Mexican assumption that essentially all weapons that end up in criminal hands in Mexico are imported illegally from the United States.

The U.S. officials say that such weapons often are the product of legitimate transactions involving the military or the national police, then are diverted as part of an illicit scam.

Luis Garfias Magana, head of National Defense Commission in the Mexican Chamber of Deputies, said the United States is ``very conscious'' of the smuggling problem and is seeking ways to combat it. A U.S.-Mexican working group on arms smuggling was created in March 1996.

At the request of Mexico, the Treasury Department's Bureau of Alcohol, Tobacco and Firearms has been trying to determine if almost 4,300 weapons confiscated in Mexico came from the United States.

Garfias Magana says loose U.S. weapons-control regulations are a big part of Mexico's problem. ``We have a federal firearms law, while in the United States it is much easier to obtain firearms,'' Garfias Magana said.

Ambassador Silva Herzog said that when U.S. officials discuss the drug problem, they often contend supply is to blame rather than the U.S. demand. ``But when we talk about illicit trafficking of arms,'' he said, ``they say it is a problem of demand.''