WASHINGTON—The Department of Homeland Security wants the U.S. military to maintain its presence at the southwest border through September 2020, a Pentagon spokesman said Friday.

Troop levels at the border, including both active-duty military and the National Guard, have at times surpassed 5,000 since President Donald Trump began deploying them there in the fall of 2018.

At the time, defense officials told the media the mission would be "months, not years."

Pentagon spokesman Major Chris Mitchell said Defense Secretary Mark Esper received the request for his review. Congress was notified about it Thursday night, a Pentagon official said.

It wasn't immediately clear how many troops will be requested to monitor the border through the fall of 2020, or whether any of them will be active duty.

Department of Homeland Security spokespersons did not immediately return requests for comment.

Trump has said the use of the military is necessary to stop illegal immigration and drug trafficking from Mexico into the United States. However, active-duty troops are barred from performing law enforcement functions inside the country under the Posse Comitatus Act, a federal law enacted in 1878 that prohibits the government from using military forces to act as a police force within its borders.

When the first troops arrived in California, Arizona and Texas last fall, their main mission was reinforcing existing border fencing by building barricades and hanging barbed wire.

NBC News reported last week that some of the troops are being used to provide wellness checks on immigrants by observing, but not touching, them from a raised platform.

Some Democratic members of Congress have warned that the expanded and extended use of the military at the border is a form of "mission creep."