(CNN) US Customs and Border Protection says it is investigating an incident at the California border with Mexico, in which agents used tear gas, pepper spray and smoke as more than 100 migrants tried to enter the country illegally.

A migrant, part of a caravan of thousands from Central America trying to reach the United States, covers his face after U.S. Customs and Border Protection (CBP) throw tear gas to the Mexican side of the fence as they prepared to cross it illegally, in Tijuana, Mexico, January 1, 2019. REUTERS/Mohammed Salem

Some of the migrants threw rocks as others lifted children over the razor-edged wire late Monday in the San Diego sector, the agency said a statement, which was based on initial reports. The confrontation continued into the early hours of Tuesday morning.

One image distributed by Reuters shows a man covering his mouth as he is surrounded by smoke. Another shows people going over the wall as armed agents stand on the US side.

Border Patrol agents hold their guns as migrants try to go over the border fence near Tijuana, Mexico.

About 150 migrants approached the fence, but about one-third of the group members turned back into Mexico when they saw Border Patrol agents, CBP estimated.

A migrant is lowered toward concertina wire on the US side of the border fence.

Others tried to enter the United States by going over or under the fence. The agency said teenagers wrapped in jackets, blankets or mats were put over the wire, as were "toddler-sized children."

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