American border officers sent tear gas into Mexico early Tuesday to drive away about 150 migrants trying to cross the border into the United States, the authorities said.
In a statement, the Customs and Border Protection agency said that the migrants tried to climb over and crawl under the border fence near San Diego and Tijuana, Mexico — the same area where American officers fired tear gas across the border late last year and where Mexico is struggling to handle thousands of migrants who have fled violence and poverty in Central America.
Early Tuesday, as migrants gathered at the border fence there, several teenagers with heavy jackets, blankets and rubber mats tried to cross or cover concertina wire at the barrier. Others began throwing rocks over the fence at the American officers, according to the statement.
The American officers also saw members of the group try “to lift toddler-sized children up and over the concertina wire,” dangerously so, it said.
At this point, the agency said, its officers used smoke, pepper spray and tear gas “upwind of the rock throwers and south of the border fence” and not toward migrants already in the United States or trying to cross at the fence line.
The officers used tear gas “only after there were rocks and there were kids involved,” said a spokesman for the agency, Andrew Meehan. “Then it became an issue of safety for the officers and frankly safety of the migrants.”
At least three volleys of tear gas were sent across the border, according to The Associated Press, which said the gas affected women and children.
In its account, Customs and Border Protection said that “no agents witnessed any of the migrants at the fence line, including children, experiencing effects of the chemical agents, which were targeted at the rock throwers.”
After the tear gas was deployed, most of the migrants fled back under the fence and away from the border, Mr. Meehan said.
Twenty-five migrants were detained, including two teenagers, the agency said.
The Trump administration’s Department of Homeland Security, which oversees Customs and Border Protection, has come under intense scrutiny for how it has treated migrants at the border and in the United States.
Rights experts have questioned whether the firing of tear gas across the border is justifiable or legal; the number of detained migrant children has soared to the highest ever recorded; and at least two children have died in United States custody.
Mr. Meehan could not immediately provide data about how often border officers use tear gas or pepper spray, citing the government shutdown’s effect on resources, and said the agency would review the incident as it does all uses of force.
“The use of those types of nonlethals is fairly common in law enforcement organizations,” he said, “but the number of uses of force for C.B.P. has been historically down year over year, in large part because of the way we review and improve our policies.”