LOS ANGELES, OCT. 5 -- For the first time since a landmark legislative effort in 1986 to control the U.S.-Mexico border, U.S. officials reported today an increase in the annual volume of illegal crossings, with more than 1 million undocumented aliens -- most of them Mexican workers -- apprehended in fiscal 1990.

Verne Jervis, a spokesman for the Immigration and Naturalization Service, said many Mexicans were using fraudulent documents to skirt the 1986 law designed to deny them jobs here. Poor economic conditions in Mexico also are adding to the flow, he said.

He confirmed the sharp reversal in border apprehensions based on preliminary estimates for the year ending Sept. 30.

Immigration groups said the turnabout proved that employer sanctions, the heart of the 1986 Immigration Reform and Control Act (IRCA), had failed and that Congress must make drastic adjustments before border barriers crumble completely.

"The growth in the labor force in Latin America is explosive," said Dan Stein, executive director of the Federation for American Immigration Reform, who wants sanctions toughened.

"We have to reopen the whole issue," said Cecilia Munåoz, senior policy analyst with the National Council for La Raza, who wants sanctions abolished.

In fiscal 1986, a record 1.6 million undocumented aliens were apprehended. President Ronald Reagan signed IRCA, which imposed heavy fines on employers who knowingly hired illegal aliens and which opened the way for legal status for more than 3 million foreigners who had lived in the United States without permission since 1982.

The law also authorized major increases in the number of Border Patrol officers and INS investigators, but this has not been fully funded because of federal budget problems.

The number of border apprehensions dropped to 1,122,067 in fiscal 1987, 940,670 in fiscal 1988 and 854,057 in fiscal 1989, but INS officials reported late last year that monthly figures were showing an upturn and immigration experts said many Mexicans were writing home to say the sanctions had few teeth.

Although affected by changes in the number of Border Patrol officers and often frequent arrests of the same individuals, border apprehensions are considered the most reliable gauge of illegal traffic. Border Patrol officials estimate that at least one undocumented alien crosses the border undetected for each one caught.

Jervis said apprehensions for the 11 months of fiscal 1990 through August totaled 954,000 and that, because about 100,000 illegals have been apprehended each month recently, INS anticipates an annual total of more than 1 million.

"We don't know if it's a trend yet or not," Jervis said. "Employer sanctions were not expected to be perfect . . . but without them, where would we be now?"

Stein's organization has promoted a proposed uniform work-authorization card that would allow employers to check the validity of residence documents as quickly as a store clerk can check the validity of a credit card.

Stein praised a bill recently introduced by Sen. Alan K. Simpson (R-Wyo.), a principal author of IRCA, that would move toward such a system as well as increase civil penalties for document fraud and add new sensors and physical barriers to the border.

In a 1988 report, Wayne A. Cornelius, director of the Center for U.S.-Mexican Studies at the University of California at San Diego, said "32 percent of the immigrant workers whom we have interviewed . . . admitted that they had purchased or used fake documents to gain employment."

Cornelius said the actual figure probably was higher because "many undocumented migrants regard the use of bogus documents as an unsavory business" and are embarrassed to admit that they have them.

Munåoz's group has opposed employer sanctions because of concern they would lead to discrimination against legal resident Hispanics seeking work. A recent General Accounting Office report confirmed such discrimination, and Munåoz said the best course is more Border Patrol officers "with better training" to end reported abuses.

Instead of employer sanctions, she favors tougher enforcement of existing labor standards for exploitative employers who often hire illegal aliens.