The United States embarked Friday on a policy of automatically rejecting asylum claims of people who cross the Mexican border illegally in a bid to deter Central American migrants and force Mexico to handle them.

President Donald Trump signed an executive order aimed at halting the flow of migrants seeking to cross into the United States without authorization, most of them requesting asylum due to violence in their home countries.

"The continuing and threatened mass migration of aliens with no basis for admission into the United States through our southern border has precipitated a crisis and undermines the integrity of our borders," Trump said in the order.

Trump used his emergency powers for the order, which critics said violates international law

protecting asylum seekers.

UNHCR, the UN human rights body, criticized Washington for not meeting obligations to help refugees.

"UNHCR expects all countries, including the United States, to make sure any person in need of refugee protection and humanitarian assistance is able to receive both promptly and without obstruction," it said.

The Geneva-based agency said that the United States was forcing migrants to turn to human smug-

glers to cross borders illegally.

The American Civil Liberties Union, Southern Poverty Law Center, and Center for Constitutional Rights together immediately filed a lawsuit challenging the order.

"Ever since the horrors of World War II, the world's nations have committed to giving asylum seekers the opportunity to seek safe haven. The Trump administration cannot defy this most elementary humanitarian principle," said

Baher Azmy, legal director of the Center for Constitutional Rights.

US officials said that as Mexico is the first safe country US-bound migrants from Guatemala, El Salvador and Honduras arrive in, the asylum claims should be presented there.

"Mexico is undoubtedly a safe country for these individuals fleeing persecution," an administration official told journalists Friday. "They should be seeking protection in Mexico."

Trump's order was explicit in wanting Mexico to deal with the problem. It said the automatic denial of asylum claims to illegal border-crossers would continue for 90 days or until there is an agreement which "permits the United States to remove aliens to Mexico."

The move came as a caravan of thousands of migrants, which Trump has branded an "invasion" of "criminals," makes its way northward through Mexico toward the border.

The US order says asylum requests will continue to be accepted from migrants who seek to cross at official US ports of entry.