U.S. border agents launched tear gas into Mexico early Tuesday to deter a group of migrants from crossing the U.S.-Mexico border at Tijuana, according to witnesses and the U.S. Government.

Clouds of the noxious gas could be seen wafting up from around the fence at the border, and one migrant picked up a canister and threw it back across the border into U.S. territory. Tijuana has become a flashpoint in the debate over U.S. immigration policy, which has been intensified by the recent deaths of two migrant children in U.S. custody and a partial U.S. Government shutdown over President Donald Trump’s US$5 billion demand to lawmakers for a wall along the border with Mexico.

A previous incident in November when U.S. agents fired gas into Mexico to disperse migrants triggered a call from Mexico for an investigation. A spokesman for Mexico’s foreign ministry did not immediately respond to a request for comment.

Late Monday, more than 150 Central American migrants approached an area of the border in Tijuana in the Playas neighborhood near the beach. Migrants said they thought security measures might be relaxed due to the New Year’s holiday.

After midnight, U.S. security personnel fired tear gas into Mexico as some migrants prepared to climb a border fence, according to witnesses.

During a second attempt, migrants began to pass youths and children over the razor wire along the fencing to the U.S. side.

U.S. Customs and Border Protection (CBP) said that the gas was aimed upwind of people throwing rocks on the Mexican side who obstructed agents from helping the children being passed over razor wire.

CBP said most of the migrants attempting to cross returned back to Mexico while 25 people were detained.  (SD-Agencies)