Thousands of migrants have set off on foot from southern Mexico in an effort to reach the United States border.
Around 7,000 people mainly from South and Central America, including thousands of children, are estimated to have joined the migrant caravan.
They left just days before US Secretary of State Antony Blinken is due to discuss how to curb mass migration with the Mexican president
Several border crossings have recently been closed due to a migrant surge.
White House national security spokesman John Kirby said US President Joe Biden and his Mexican counterpart, Andrés Manuel López Obrador, shared concern about the "dramatic" increase in migrants crossing their joint border.
The number of people apprehended at the US southern border exceeded two million both in the 2022 and the 2023 fiscal years.
In September 2023 alone, US Border Patrol apprehended more than 200,000 migrants crossing the US-Mexico border unlawfully, according to US Homeland Security figures.
The latest migrant caravan left from the southern Mexican city of Tapachula, near the country's southern border with Guatemala, on Christmas Eve.
Its leaders carried a banner reading "Exodus from poverty".
Local media said that most of the migrants were from Cuba, Haiti and Honduras, but some came from as far away as Bangladesh and India.
Many said that they had decided to join the caravan after waiting for months for transit permits.
The migrant rights activist Luis García Villagrán, who is accompanying the caravan, said joining the mass trek north was a last resort for many of the migrants who had been stuck in Tapachula.
"The problem is that the southern border [with Guatemala] is open and 800 to 1,000 people are crossing it daily. If we don't get out of Tapachula, the town will collapse.
"We tell the Mexican state that it has left us no other option but to take the coastal highway and walk as far as we can get," he said.
The migrants covered some 15km on the first morning, after setting off at dawn on 24 December.
One Honduran migrant said he had left his home country to escape a criminal gang which had threatened to kill him.
José Santos told Reuters news agency: "I was scared so I decided to come to Mexico hoping I'll be allowed to go to the US."
On Friday, Mexican President Andrés Manuel López Obrador said he was willing to work again with the US to address concerns about migration.
The Mexican leader is due to meet the US secretary of state on Wednesday.
Their meeting comes at a time when the surge in immigration is a hot political topic in the US with pressure mounting on President Biden to stem the flow across the US southern border.
