Image copyright Reuters Image caption The migrants believed security may be more relaxed on New Year's Day

US agents have fired tear gas over the border into Mexico at migrants trying to enter the country illegally.

Around 150 Central Americans tried to make the crossing near the town of Tijuana to the south of California on New Year's Day.

One US official described the migrants as a "violent mob".

It comes as the US federal government remains shut down as President Donald Trump and Congress argue over funding for his proposed border wall.

Image copyright AFP/Getty Image caption Around 150 Central American migrants tried to make the crossing

Image copyright AFP/Getty Image caption The US says it was not aiming tear gas at the migrants

The US said security forces used tear gas after migrants threw rocks, and that they were deliberately aiming upwind of rock-throwers who they say hindered agents from helping migrants who were passing children over razor wire.

But the Associated Press reports rocks were only thrown after agents fired tear gas, and a Reuters witness says one migrant was struck by what seemed to be a tear gas canister.

Image copyright Reuters Image caption Some threw the canisters back over the border wall into the US

Image copyright Reuters Image caption A Reuters witness said one migrant was hit by what looked like a tear gas canister

US Homeland Security spokeswoman Katie Waldman called the group a "violent mob", and said agents used "the minimum force necessary to defend themselves".

In a statement, Ms Waldman called on Congress to "fully fund the border wall".

Image copyright AFP/Getty Image caption Some migrants reportedly tried to pass children over the fence into the US

Image copyright Reuters Image caption Migrants used a treatment to counteract the tear gas effects

US Customs and Border Protection said 25 people were detained, including two teenagers. The majority of migrants returned to Mexico.

A spokesperson for Mexico's foreign ministry said the government "regrets the events" near Tijuana, and called on both sides to respect the law.

Image copyright Reuters Image caption Immigration is a major flashpoint in US politics, with President Donald Trump insisting the US needs a border wall

Image copyright Reuters Image caption US Homeland Security spokeswoman Katie Waldman called the group a "violent mob", and called for border wall funding

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