MEXICO CITY, Dec. 28 (Xinhua) -- As the White House mulls whether or not to seal off the U.S. southern border, Mexico's President Andres Manuel Lopez Obrador on Friday reacted cautiously, saying the decision was a "domestic issue" for the United States.

"We have not issued an opinion on the matter because it is a domestic issue for the U.S. government and we prefer to abstain" from commenting, Lopez Obrador said during his daily press conference.

"We are always looking to maintain very good ties with the United States," he added.

Earlier in the day, U.S. President Donald Trump raised the prospect of sealing off the border completely unless Democratic-controlled Congress agrees to fund the controversial border wall.

"We build a Wall or ... close the Southern Border," Trump said via Twitter.

To pressure lawmakers, Trump last Saturday sparked a partial shutdown of the federal government.

Trump wants 5 billion U.S. dollars for the wall, which was a central promise of his 2016 presidential campaign.

Democrats have agreed to approve 1.3 billion U.S. dollars to bolster security at the border, but have long opposed the building of a wall.