US authorities on Tuesday started returning people to Mexico after they had lodged their asylum applications, a first under President Donald Trump's controversial "Remain in Mexico" policy.

"The United States has begun implementing the Migrant Protection Protocols," the US embassy in Mexico said in a statement. "This action is a response to the illegal migration crisis the United States is facing on its southern border."

Mexican authorities confirmed that the first asylum seekers had arrived in the country. Last week, Mexican President Manuel Lopez Obrador said the Trump administration had pledged to return some 20 people per day via the Tijuana border crossing.

"The United States has decided unilaterally to ask them to wait in Mexico while their asylum claims are processed," Rodulfo Figueroa, who heads the Mexican immigration office in Tijuana, said. But the claims could take months, if not years, to process due to a backlog of some 800,000 asylum requests.

Read more: In Mexico, migrants must choose between bad and worse

'Bad idea'

For Trump, the policy is aimed at deterring irregular migration to the US. The White House has argued that there is a "humanitarian and security crisis." However, many of those arriving at the border have fled gang violence and extreme poverty in Central America, and are seeking sanctuary. The policy won't apply to Mexicans seeking asylum.

Critics have hit out at Trump over the policy, with Melissa Crow of the US-based Southern Poverty Law Center calling it a "horribly bad idea."

"It's not safe for many, if not most, of these asylum seekers to wait in Mexico. Lots of them are fleeing cartel violence and domestic abusers," Crow told the Agence France-Presse news agency. "There are examples of being pursued by their persecutors while waiting in Mexico."

The policy implementation comes as Trump recovers from a crushing government shutdown defeat. He vowed to not approve a budget until it included $5.7 billion (€5 billion) for a border wall, a contentious campaign pledge he has yet to fulfill.

Read more: Latin America: A year of movement

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ls/se (AFP, dpa)