By Jorge Cabrera

AGUA CALIENTE, Guatemala (Reuters) - Up to 3,000 migrants crossed from Honduras into Guatemala on Monday on a trek northward, after a standoff with police in riot gear and warnings from Washington that migrants should not try to enter the United States illegally.

The crowd more than doubled in size from Saturday, when some 1,300 people set off from northern Honduras in what has been dubbed "March of the Migrant," an organizer said. The migrants plan to seek refugee status in Mexico or pass through to the United States.

Reuters could not independently verify the number of participants, but images showed a group carrying backpacks and clogging roads near the border, some waving the Honduran flag.

The impoverished nations of Central America, from which thousands of migrants have fled in recent years, are under mounting pressure from U.S. President Donald Trump's administration to do more to curb mass migration.

"We are seriously concerned about the caravan of migrants traveling north from Honduras, with false promises of entering the United States by those who seek to exploit their compatriots," the U.S. Embassy in Honduras said in a statement on Sunday evening.

U.S. Vice President Mike Pence last week called on presidents in the region to tackle the issue, saying Washington would be willing to help with economic development and investment in return.

Guatemala said in a statement on Sunday that it did not promote or endorse "irregular migration."

Rows of Guatemalan police in black uniforms, some wearing helmets and shields, initially blocked migrants from reaching a customs booth, Reuters images showed. It was not clear how long the standoff lasted, but the group was ultimately able to cross, said march organizer Bartolo Fuentes, a former Honduran lawmaker.

A police official on site said all Central Americans could pass freely through the region as long as they complied with migration control.

"We're going to drop in on Donald Trump. He has to take us in," said Andrea Fernandez, 24, who left Honduras with a newborn baby, a 5-year-old daughter and 7-year-old son because she said she could not find work and feared for their safety.

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Mexico's migration institute said in a statement on Monday that march participants would need to follow immigration rules to enter the country, without specifying the criteria.

"The law does not provide for any permission to enter the country without meeting the requirements, and then go on to a third country," the government agency said.

(Reporting by Jorge Cabrera; Additional reporting by Gustavo Palencia in Tegucigalpa; Writing by Daina Beth Solomon; Editing by Peter Cooney)