REUTERS - U.S. authorities closed the world’s busiest land border crossing on Tuesday after a shootout between suspected Mexican human traffickers and U.S. agents, U.S. officials said.

"The port is closed and will remain closed for several hours," U.S. Customs and Border Patrol spokeswoman Angelica Decima said after the incident at the congested San Ysidro crossing between the Mexican city of Tijuana and San Diego.

The suspected smugglers shot across busy lines of traffics at U.S. agents when they tried stop three vans packed with about 70 illegal immigrants from crossing into the United States, the officials said.

The agents returned fire, and three people in the vans and a motorist were wounded in the gunfight, said Lauren Mack, a spokeswoman for U.S. Immigration and Customs Enforcement.

All the illegal immigrants were arrested and taken into custody and the crossing, a major smuggling corridor for narcotics and illegal immigrants, was shut while police carried out an investigation.

Some 90 million people a year use the California-Mexico land border crossings, with almost half the traffic going through San Ysidro alone.