WASHINGTON/ESQUIPULAS, Guatemala (Reuters) - The organizer of a migrant caravan from Honduras was detained in Guatemala on Tuesday as the U.S. government threatened to withdraw aid from both countries and El Salvador if the flow of migrants north to the United States was not stopped.

Up to 3,000 migrants, according to organizers’ estimates, crossed from Honduras into Guatemala on a trek northward after a standoff on Monday with police in riot gear.

The Honduran Foreign Ministry called on its citizens not to join the group. The government “urges the Hondurans taking part in this irregular mobilization not to be used by a movement that is clearly political,” it said.

Honduran president Juan Orlando Hernandez said in a public address on Tuesday evening some Hondurans in the caravan had already returned home and the government was preparing to support them. He did not specify how many had turned back.

Over the border, Guatemalan police officers detained Bartolo Fuentes, a former Honduran lawmaker, from the middle of the large crowd that he and three other organizers had led from San Pedro Sula, Honduras, since Saturday.

The moves followed comments by U.S. President Donald Trump that his administration would halt aid if the Central American governments did not act, his latest effort to demonstrate his tough stance on immigration.

The Honduran security ministry said Fuentes had been detained because he “did not comply with Guatemalan immigration rules” and would be deported back to Honduras in the coming hours.

Security officials at the Honduran border with Guatemala in Agua Caliente blocked the road to prevent another much smaller group from getting through, television images from the border showed.

“We can’t attend to people en masse. People are going through one by one,” police spokesman Alex Madrid said in a radio interview.

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Guatemala’s government said it did not have official figures for how many migrants from the caravan had already crossed the border.

Adult citizens of Guatemala, Honduras, El Salvador and Nicaragua need only present national identity cards to cross each others’ borders. That rule does not apply when they reach Mexico.

The local offices of the United Nations High Commissioner for Human Rights said in a statement on Tuesday evening they were worried about the safety of migrants in the caravan, noting the group included women, children and senior citizens.

“NO MORE MONEY”

Trump took to Twitter on Tuesday to express his annoyance at the caravan, which follows a similar event in May that ultimately led to hundreds of migrants either seeking asylum in the United States or remaining in Mexico.

“The United States has strongly informed the President of Honduras that if the large Caravan of people heading to the U.S. is not stopped and brought back to Honduras, no more money or aid will be given to Honduras, effective immediately!” Trump wrote.

Trump expanded his threat in a later tweet to include Guatemala and El Salvador.

“We have today informed the countries of Honduras, Guatemala and El Salvador that if they allow their citizens, or others, to journey through their borders and up to the United States, with the intention of entering our country illegally, all payments made to them will STOP (END)!” he said.

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U.S. Vice President Mike Pence drove home the point, saying he spoke to Hernandez and Guatemalan President Jimmy Morales and warned them to help protect U.S. borders, adding “no more aid if it’s not stopped!”

The strong words could encourage Honduras to move closer to China amid intensified efforts by Beijing to win recognition from Central American countries aligned with Taiwan.

Honduras is one of a dwindling number of countries that still have formal diplomatic ties with Taiwan, an island nation off the Chinese coast that Beijing views as a renegade province.

Hernandez said last month cuts in U.S. support for Central America would only hinder the country’s ability to stem illegal immigration. He welcomed China’s growing diplomatic presence in the region as an “opportunity.”

Pence told Central American countries last week the United States was willing to help with economic development and investment if they did more to tackle mass migration, corruption and gang violence.

GROWING GROUP

The migrants in the group making its way north plan to seek refugee status in Mexico or pass through to the United States, saying they are fleeing poverty and violence.

“What Trump says doesn’t interest us,” organizer Fuentes said in an interview shortly before his arrest. “These people are fleeing. These people are not tourists.”

Widespread violence and poverty prompt thousands of Central Americans, mainly from El Salvador, Honduras and Guatemala, to make the arduous journey north toward Mexico and the United States in search of a better life.

Trump ran for president in 2016 on promises to toughen U.S. immigration policies and build a wall along the 2,000-mile(3,220-km) border with Mexico.

Illegal immigration is likely to be a top issue in Nov. 6 U.S. congressional elections, when Democrats are seen as having a good chance of gaining control of the House of Representatives from Trump’s fellow Republicans.