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U.S. authorities fired tear gas into Mexico during the early hours of the new year to repel about 150 migrants who tried to breach the border fence in Tijuana.

U.S. Customs and Border Protection said in a statement that about 45 migrants got through the first layer of fencing but turned back when they saw the border patrol's presence early Tuesday. CBP said migrants began to throw rocks at agents from the Mexican side while others tried to cross the concertina wire, including passing "toddler-sized children" over the wire.

Border patrol officers responded by firing smoke, pepper spray and tear gas at the rock throwers in Mexico, distinguishing them from those who were trying to cross the fence line, CBP said.

"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 further away," the statement said.

However, an Associated Press photographer saw at least three volleys of gas launched onto the Mexican side near Tijuana's beach that affected the migrants, including women and children, as well as journalists. The AP saw rocks thrown only after U.S. agents fired the tear gas.

The AP photographer was on the Mexican side of the border, while border patrol agents who provided the information for the CBP statement were on the U.S. side.

The incident took place between a highway and the border fence. The area is flat near the highway, then slopes sharply downward closer to the fence, where most of the migrants were clustered.

U.S. authorities also had a helicopter flying over the scene during the incident so presumably the government has video that could show what was happening on both sides.

The agency said 25 migrants were detained, while others crawled back into Mexico through a hole under the fence.

CBP said that under its use of force policy, the incident would be reviewed by its Office of Professional Responsibility.

Migrants who spoke with AP said they arrived in Tijuana last month with the caravan from Honduras.

The caravan left Honduras in mid-October and grew to more than 6,000 members during its month-and-a-half trek north. It has been a constant target of President Donald Trump, who referred to it frequently before U.S. midterm elections in November.

Many of the migrants are waiting in Tijuana for a chance to apply for asylum in the U.S., but there was a backlog before the caravan's arrival and the wait is expected to be many months. Others have found jobs in Mexico and tried to settle there.

In a previous incident, U.S. agents launched tear gas across the border after some migrants tried to breach the border following a peaceful march in Tijuana on Nov. 26. Hundreds of migrants who were downwind of the gas were affected.

Trump has been locked in a fight with Congress over funding for the border wall that he wants to build. The stalemate has led to a partial government shutdown.