Mexicans seeking asylum in the United States could be sent to Guatemala under a bilateral agreement signed by the Central American nation last year, according to documents sent to US asylum officers in recent days and seen by Reuters.

In a January 4 email, field office staff at the US Citizenship and Immigration Services (USCIS) were told Mexican nationals will be included in the populations "amenable" to the agreement with Guatemala.

The controversial agreement, brokered last July between the administration of Republican President Donald Trump and the outgoing Guatemalan government, allows US immigration officials to force migrants requesting asylum at the US-Mexican border to apply for protection there.

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Trump has made clamping down on undocumented migration a top priority of his presidency and a major theme of his 2020 re-election campaign. His administration penned similar deals with Honduras and El Salvador last year.

Democrats and pro-migrant groups have opposed the move and contend asylum seekers will face danger in Guatemala, where the murder rate is five times that of the US, according to 2017 data compiled by the World Bank. The country's asylum office is tiny and thinly staffed and critics have argued it does not have the capacity to properly vet a significant increase in cases.

Guatemalan President-elect Alejandro Giammattei, who takes office this month, has said he will review the agreement.

Acting Deputy US Department of Homeland Security (DHS) Secretary Ken Cuccinelli said in a tweet in December that Mexicans were being considered for inclusion under the agreement.

USCIS referred to Cuccinelli's tweet, and US Customs and Border Protection and Mexico's foreign ministry did not immediately respond to requests for comment.

Unaccompanied minors cannot be sent to Guatemala under the agreement, which currently applies only to migrants from Honduras, El Salvador and Mexico who entered the US after November 19, 2019, according to the guidance documents. Exceptions are made only if the migrants can establish that they are "more likely than not" to be persecuted or tortured in Guatemala based on their race, religion, nationality, membership in a particular social group or political opinion.

Numbers of Central American migrants apprehended at the border fell sharply in the second part of 2019 after Mexico deployed National Guard troops to stem the flow, under pressure from Trump.

Overall, border arrests are expected to drop again in December for the seventh straight month, a Homeland Security official told Reuters last week, citing preliminary data.

The US government says another reason for the reduction in border crossings is a separate programme, known as the Migrant Protection Protocols, that has forced more than 56,000 non-Mexican migrants to wait in Mexico for their US immigration court hearings.

With fewer Central Americans at the border, US attention has turned to Mexicans crossing irregularly or requesting asylum. Around 150,000 Mexican single adults were apprehended at the border in fiscal 2019, down sharply from previous decades but still enough to bother US immigration hawks.