"If they don't, or if we don't make a deal with Congress, the border's going to be closed, 100 percent," Trump told reporters in the Oval Office, doubling down on a threat he first made at the weekend.

Trump went on to say he was ready to "close large sections of the border, maybe not all of it."

Delays were already rising at several key border crossings as Trump ratcheted up pressure on Democrats in Congress to toughen US laws against illegal immigrants, threatening to hurt the US economy in the name of border security.

"It is a national emergency on the border," Trump said.

If Mexico does not stop migrants from transiting across its territory, and if Congress does not act, he said, "the border's going to be closed, 100 percent."

"Sure it's going to have a negative impact on the economy," he said. "Security is more important to me than trade."

Trump said Democrats were blocking reforms for political reasons but could fix the problem "in 45 minutes".

He was not specific, but Homeland Security Secretary Kirstjen Nielsen has asked Congress to give border officials the power to forcibly repatriate Central American illegal immigrants, including children who arrived alone, back to their countries.

The threat to close the border, which handled some $612-billion worth of trade in 2018, sent shivers through the economy and drew warnings from allies of the president.

"Closing down the border would have potentially catastrophic economic impact on our country, and I would hope we would not be doing that sort of thing," Senate Majority Leader Mitch McConnell said.

His call was backed by the Chamber of Commerce, which said in a statement that while it backed immigration reform, closing the border "would inflict severe economic harm on American families, workers, farmers and manufacturers across the United States."

AFP