Crimes against American tourists on Mexican highways have fallen sharply since the government beefed up security late last year in response to U.S. complaints, U.S. Embassy officials said today.

This decline, and a new Mexican program to help foreigners with their legal problems, have led the embassy to tone down its public statements about the danger to visitors here, the U.S. officials said. The embassy and State Department are opposing an unusual effort in Congress to require that the U.S. government issue a formal travel advisory to notify Americans about the risks of visiting Mexico.

"More than 4 million Americans visit Mexico each year, and relatively few of them encounter problems," U.S. Ambassador John Gavin said in a speech Thursday to the World Business Council in San Antonio, Tex. "We don't believe conditions warrant a travel advisory at this time."

The question of tourist safety has become a significant irritant in U.S.-Mexican relations in the past year. U.S. complaints about crimes against visitors were interpreted here as a threat to the vital tourism industry and were labeled a "defamation campaign" against Mexico. U.S. tourists spent about $2 billion here last year, and some resort regions depend almost exclusively on tourist dollars.

An embassy official acknowledged that earlier U.S. statements about dangers to tourists had opened up "a Pandora's box" and contributed to exaggerated fears about traveling in Mexico. Those worries were reinforced by the much publicized killing of a U.S. Drug Enforcement Administration agent in February in Guadalajara, which triggered the interest in Congress.

The issue originally gained prominence last September, when Gavin publicly called attention to a series of crimes against American tourists on Mexican highways. From March until September of last year, six U.S. citizens were slain in highway incidents, most of which involved armed robbery.

Gavin also said at the time that the U.S. government was considering issuing a travel advisory for Mexico.

Embassy officials acknowledged that, on a "volume basis," the level of crime here has been "relatively low." And while Gavin did not say so at the time, one of his main reasons for airing the possibility of a travel advisory in September was to pressure the Mexicans to speed up, or even begin, investigations of past crimes against U.S. citizens, embassy officials said.

"What we have been concerned with primarily is not so much the number of crimes committed against Americans but the less than vigorous investigation and prosecution of these crimes," Gavin said Thursday.

The U.S. pressure has borne some fruit, although it is too early to tell whether improvement will be permanent. In particular, increased patrols by police cars and Mexico's bright green highway rescue vehicles, called "Green Angels," have cut down on roadway crimes. There has not been a serious highway crime against an American tourist at least since February, an embassy spokesman said.

The Mexican national attorney general's office opened a new program in February to assist foreign crime victims. The U.S. Consulate has passed more than 50 unresolved cases to that office, and some progress has been made, Consul General Larry Lane said.