The US has threatened to arrest around 100 Central American migrants if they try to sneak in from the US-Mexico border where they have gathered, prompting President Donald Trump to order troop reinforcements on the frontier.
Two busloads of migrants arrived Tuesday in the border city of Mexicali -- which sits opposite Calexico, California -- and paused for a rest stop before continuing on to Tijuana.
They are part of a caravan of more than 1,000 people who set out from Mexico's southern border on March 25.
Media coverage of the caravan triggered a flurry of furious tweets from Trump, who ordered thousands of National Guard troops to the border and called on Mexico to stop the migrants.
As the issue exploded into a diplomatic row, organizers of the caravan quietly broke it up.
The Department of Homeland Security issued a stark warning Wednesday to the first 120 who arrived, including some 50 minors.
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"If you enter our country illegally, you have broken the law and will be referred for prosecution," DHS Secretary Kirstjen Nielsen said.
Nielsen said those making false immigration claims or helping others to do so would also be prosecuted.
Around 600 migrants are still traveling more or less together by hopping trains and taking buses.
The first two buses will be followed by three more, said organizer Irineo Mujica of the migrant rights group People Without Borders.
Around 200 migrants will seek asylum in the United States, fleeing brutal gang violence or political persecution in their home countries, he said. A handful of them have already been granted US asylum, Mujica told AFP.
Others may try to cross into the US illegally.
"It's hard to say, they don't tell you (they plan to sneak over the border). Some will, but probably not right now. There's too much pressure because of the troops Trump sent to the border," he said.
The caravan is a yearly event held since 2010 around Easter. Activists say its main goal is to raise awareness of the perils migrants face, not actually to cross the border.
Ties between the United States and Mexico have been strained since Trump won the presidency in 2016 on the back of a campaign heavy on anti-Mexican rhetoric and promises to build a wall on the two countries' border and make Mexico pay for it.