Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE is expected to sign an order sending up to 800 troops to the U.S.-Mexico border amid President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE’s ongoing focus on a migrant caravan traveling to the United States, according to multiple reports.

Citing unidentified U.S. officials, reports said Mattis could sign the order as soon as Thursday to send hundreds of U.S. service members to help with logistical support such as providing tents and fencing.

A spokesman for the Pentagon later on Thursday said it expected to get a request for assistance from the Department of Homeland Security.

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"The Department of Defense continues to monitor events along the Southwest U.S. border, including the status of the migrant caravan heading north through Mexico," Pentagon spokesperson Capt. Bill Speaks said in the statement.

"We anticipate receiving a request for assistance (RFA) from the Department of Homeland Security and are currently working with DHS to determine the specifics of our support to Customs and Border Protection (CBP)."

The Pentagon previously had said it had received no official request from the White House to send more troops to the border.

Since last week, Trump has said he plans to bring in the military to reinforce the southern border as a group of thousands of migrants makes it from Central America.

The caravan is currently traversing Mexico, and Trump administration officials have vowed not to allow the group to cross into the United States.

On Thursday morning, Trump tweeted that he is “bringing out the military for this National Emergency.”

“They will be stopped!” he added.

In April, Trump directed Mattis to send 4,000 National Guardsmen to the U.S.-Mexico border amid frustrations at the lack of progress on his proposed wall.

About 2,100 Guardsmen are currently providing technical assistance at the border.

This story was updated at 5:21 p.m.