US border agents have reportedly fired tear gas into Mexico to push back against what one official called a "violent mob" of migrants as they rushed the US-Mexico border near Tijuana.

Reuters reports that clouds of the noxious gas could be seen wafting up from around the fence at the border. One migrant picked up a canister and threw it back into U.S. territory.

More than 150 Central American migrants approached an area of the border in Tijuana in the Playas neighborhood near the beach late on Monday.

Migrants said they thought security measures might be relaxed due to the New Year's holiday.

U.S. security personnel fired tear gas into Mexico after midnight as some migrants prepared to climb a border fence, according to the Reuters witness. During a second attempt, migrants began to pass youths and children over the razor wire along the fencing to the U.S. side.