LOS ANGELES, AUG. 3 -- Amid warnings about impending fines for employers who hire undocumented workers, the Border Patrol announced today that it arrested more illegal aliens in the border sector south of San Diego last month than in any previous July.

The 63,061 apprehensions in July topped even last July's record 59,182. It marked the first time this year that arrests in the Chula Vista sector, traditionally the busiest place along the Mexican-American border, exceeded the total for the same month in 1986.

Harold Ezell, western regional commissioner of the Immigration and Naturalization Service, noting the increase, said his officers this month will begin citing employers found to knowingly hire workers who have no proof of legal U.S. residency. If warnings go unheeded, he said, agents will arrest employers under the new immigration law that calls for fines, and in severe cases, jail terms for violators.

When the law was signed in November, the Mexican news media reported rumors of mass deportations, and illegal border traffic declined sharply. In June, however, illegal immigration south of San Diego began to climb again, a trend that accelerated last month. INS said July apprehension figures for much of the rest of the Mexican border were not available yet, but total border arrests from May to June increased more than 24,000.

Gene Smithburg, assistant chief Border Patrol agent for the Chula Vista sector, said agents attributed the rapid increase there to an easing of fears in Mexico about deportations and to a healthy southern California job market that is made more attractive by Mexico's economic crisis. He said the numbers will decrease only when the new law is enforced.

Smithburg said the July increase marked the first time since October 1986 that the patrol had seen a monthly increase from the previous year.