MAPASTEPEC, Mexico (Reuters) - Tempers frayed among hundreds of mostly Central American migrants gathered on Wednesday in southern Mexico, delayed as Mexican officials sought to slow down the U.S.-bound flow that President Donald Trump is determined to turn back.

Migrants from Central America, waiting to begin their process to get their humanitarian visas to cross the country on their way to the United States, are seen outside an improvised shelter in Mapastepec, in Chiapas state, Mexico April 3, 2019. REUTERS/Jose Torres

Since last week Trump has repeatedly threatened to close down the U.S.-Mexico border if Mexican officials do not do more to thwart the migrants, potentially harming tens of billions of dollars in trade, but has also praised Mexican efforts following his outbursts.

The Mexican government has vehemently denied changing policy in response to threats, but has appeared to slam the brakes on its practice of awarding so-called humanitarian visas that allow migrants from other countries to pass freely within its borders.

Without such papers, they are vulnerable to harassment and deportation from officials.

As many as 1,500 men, women and children traveling in a large group or caravan from Honduras, El Salvador, Nicaragua, and Cuba were gathered in the town of Mapastepec in Chiapas state, unable to obtain the temporary visas.

Mexico’s immigration institute said it would prioritize giving the visas to vulnerable groups including the elderly and unaccompanied minors, while offering transport home for others.

Officials were issuing very few of the documents, migrants said, and frustrations were mounting as a result.

“It’s been hard for me to get here because there aren’t any visas,” said Cuban migrant Yuremi Garcia, who had traveled without papers from the southern Mexican border a few hours south to a crowded sports ground in Mapastepec, converted into a temporary shelter.

Garcia said he was tired of waiting and had decided to continue northwards together with others, despite the risk that Mexican authorities would deport them.

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At the border town of Tapachula, near the southern tip of Mexico bordering Guatemala, another group included people from Sri Lanka, Congo and Haiti, a federal official said. Some migrants said Mexican officials had slowed down the process of awarding the visas or denied them outright without providing any explanation.

A small group lashed out at border officials in Tapachula on Tuesday over the delays, throwing rocks and breaking windows of a local migration institute building.

Edgar Corso, an official with Mexico’s human rights commission, told Reuters that some 45 complaints filed with the commission by Cuban migrants since March 15 allege unreasonable delays in awarding the visas.

He said the attack on the migration office has also been formally documented.

Last week, Mexican President Andres Manuel Lopez Obrador, who has steadfastly avoided any public confrontation with Trump, said he would help ensure more orderly migration.

In December-February, his administration’s first three months, Mexico sent home 19,360 migrants, 17 percent fewer than a year earlier, data from the National Migration Institute show.

In response to Trump’s pressure, the government has been providing daily updates to U.S. officials on how it is acting more aggressively to halt migration flows, and providing specific numbers on how many people are being apprehended, a senior White House official said.

“They’ve shown that they are increasing what they are doing,” the official said on Tuesday, asking not to be named in order to speak freely.

U.S. Homeland Security Secretary Kirstjen Nielsen and officials at the State Department have been in regular contact with Mexican officials about their efforts, the official said.

Lopez Obrador told reporters on Wednesday at his regular morning news conference that his administration was “acting with an abundance of prudence,” saying a border shutdown was in nobody’s interest.

“We are looking to ensure that the law is respected,” he added.