MEXICO CITY, JULY 25 -- An alert on the U.S.-Mexican border for suspected Islamic Jihad terrorists has highlighted a growing problem of illegal entry into the United States by non-Mexicans who use this country as a springboard.

The alert went out to U.S. Border Patrol units two weeks ago, based on intelligence reports received July 8 that a hit squad might try to assassinate an American law enforcement official, according to Silvestre Reyes, the Border Patrol chief in McAllen, Tex.

He said the intelligence information, from sources he would not identify, warned that 10 persons of several nationalities -- possibly a "hit team" of the pro-Iranian terrorist group, Islamic Jihad (holy war) -- was making its way to the border from Mexico City.

A spokesman for Mexico's Interior Ministry said he had no information on the matter. The Mexican immigration department had no comment.

Islamic Jihad has taken responsibility for many of the kidnapings of foreigners -- including some of the nine Americans -- currently held hostage in Lebanon, and its members have committed suicide bombings against U.S. installations in Lebanon, including two against U.S. Embassy buildings in Beirut and the October 1983 truck-bombing of the U.S. Marine headquarters at Beirut International Airport.

Reyes said that although no terrorist actions have been carried out so far, the Border Patrol was taking the warning seriously. "We cannot afford to discount that type of information," he said in a telephone interview. "We're taking extra precautions."

The prospect that terrorists might use the traditional infiltration routes of Mexican illegal aliens to enter the United States undetected has heightened concerns in recent years about the overall problem of non-Mexican foreigners who cross the border illegally.

"The threat to security is obvious when you have people crossing the border who are not inspected," said Border Patrol supervisor Dean Thatcher by telephone from San Diego.

Arrests of non-Mexicans by the U.S. Border Patrol have risen sharply in the last several years, although they remain a small percentage of the total. From 1982 to 1986, arrests of non-Mexicans doubled from 24,300 to nearly 49,000, according to statistics of the U.S. Immigration and Naturalization Service. Last year, the INS says, people of 76 nationalities were caught crossing the border illegally.

The majority of the non-Mexican illegal immigrants come from Central America, INS officials say. But others come from as far away as Western and Eastern Europe, the Middle East, Africa and even mainland China.

"I view non-Mexican illegal immigration as being a much more serious problem" than border crossings by undocumented Mexicans, said Michael Trominski, the INS district director here. Of the approximately 1.6 million illegal immigrants arrested on the U.S.-Mexican border last year -- a total that may include multiple arrests of the same person -- at least 95 percent were Mexicans, he estimated.

U.S. Border Patrol authorities estimate that they catch roughly 40 percent of those who cross the 2,000-mile border illegally, including non-Mexicans.

"Our Hong Kong office says the word on the street is that Chinese can go to the U.S. for $25,000," said Trominski. "Some of the routes they take are unbelievable," he added, citing stops in countries such as Australia, Canada and Central American states.

According to INS and Border Patrol officials, investigators are looking into networks that function as modern "underground railroads" for Poles and Yugoslavs, who arrive in Mexico on tourist visas and are then smuggled across the U.S. border. The smugglers here are usually the same Mexican guides called "coyotes" who have operated for decades, the officials say.

Deportations of illegal aliens from Mexico are up, and immigration authorities have stepped up efforts to spot counterfeit passports and Mexican and U.S. visas at ports of entry. Last year, according to a report by the Mexican director of immigration services, more than 120,000 foreigners were deported for having fake documents or residing here illegally.

Mexican authorities also have shown increased concern about the possibility of transit through this country by terrorists. In one incident that reflected heightened concerns here about potential illegal immigrants from the Arab world, Mexican authorities in June 1986 detained a group of Middle Easterners -- including 11 Lebanese, six Syrians and a Turk -- even though "The threat to security is obvious when you have people crossing the border who are not inspected."

-- Border Patrol official Dean Thatcher

they had entered the country legally to attend the World Cup soccer tournament.

They were arrested when they turned up in a hotel about 60 miles south of the U.S. border in the town of Sabinas, where no matches were scheduled. Although authorities had been tipped that the group appeared to be planning to enter the United States illegally, no evidence of any terrorist activity emerged. The members of the group were later brought back to Mexico City and allowed to leave the country.

According to Border Patrol chief Reyes in McAllen, his agents have arrested "some" terrorism suspects in the past and turned them over to "another agency." He declined to elaborate, nor would other INS and Border Patrol officials who were interviewed.

In the McAllen sector, which covers nine south Texas counties, Border Patrol agents last year arrested more than 16,000 non-Mexican illegal immigrants, compared to 4,200 five years ago, Reyes said. Most of those arrested hailed from Latin American or Caribbean countries, but others came from China, Bangladesh, Zaire and Czechoslovakia, Reyes said. Also included were 12 Iranians, three Jordanians, two Lebanese, two Syrians, nine Israelis and a Libyan.

Aside from national security concerns, the illegal entry of non-Mexicans into the United States raises administrative problems for U.S. authorities. Unlike Mexicans, an estimated 90 percent of whom return to their country when they are caught, the other foreigners cannot simply be sent back across the border, and detaining and repatriating them becomes expensive.

In El Salvador, where many of the arrested non-Mexicans come from, stories are told of undocumented workers who make their way to the United States and work illegally for a couple of years, then turn themselves in to immigration authorities -- often around Christmas or some other holiday -- and get a free plane ride home for a vacation before repeating the whole process.