Four foreign nationals, whose names match those on the terror watch list, have been stopped trying to enter the U.S. via the southern border since October, a congressional aide familiar with Customs and Border Protection (CBP) information told Fox News.

That information shows that three migrants from Yemen and one from Serbia were picked up at the southern border by U.S. Border Patrol since the beginning of the fiscal year in October.

DHS CHIEF SAYS BORDER NUMBERS ON TRACK TO BE HIGHEST IN 20 YEARS, CONCEDES SITUATION IS 'DIFFICULT'  

The Serbian national was encountered on Jan. 28, amid an increasing surge in migrant numbers in recent months. The apprehensions were first reported by Axios.

A CBP spokesman said in a statement that security efforts at the border "are layered and include multiple levels of rigorous screening that allow us to detect and prevent people who pose national security or public safety risks from entering the United States."

"DHS works with our international partners to share intelligence and other information, including to prevent individuals on the terrorist watchlist from entering the United States. CBP adjudicates individuals encountered at and between our ports of entry against several classified and unclassified databases to determine if they pose a threat to national security, consistent with the law," the spokesman said. "While encounters of known and suspected terrorists at our borders are very uncommon, they underscore the importance of the critical work our agents carry out on a daily basis to vet all individuals encountered at our borders."

Republicans have been warning about the risk of terrorists getting through the border amid the surge in numbers and the rollback of a number of Trump-era protections by the Biden administration.

CBP encountered more than 100,000 migrants in February, with the Department of Homeland Security admitting that the numbers CBP are facing are "overwhelming."

Republicans have blamed that spike on Biden administration policies, and warned that loosening border security could lead to terrorists or those on watch lists getting into the U.S.

House Minority Leader Kevin McCarthy said on Monday, during a visit to the border, that Border Patrol agents had been warning lawmakers about foreign nationals on the terror watch list getting in.

"You saw it in their eyes," McCarthy said, according to The Washington Post. "They talked about, ‘They’re on the list.’

McCarthy said migrants were coming from Yemen, China, Sri Lanka and Iran. Border Patrol agents arrested 11 Iranian citizens in Arizona last month.

Rep. John Katko, the ranking member on the House Homeland Security Committee, recounted a similar conversation with a Border Patrol official.

"And he said some of them are on the list. And I said list? And he said, 'yeah, the terror watch list.' And I was stunned and I've never heard that before," he said on "America's Newsroom."

"They said they've seized not just people from Central and South America, but even people from Yemen and places in the Middle East that are terrorism hotbeds," he said. "So this is a very serious problem because well beyond the unaccompanied minor and it started because this administration changed the policies and basically put an open door sign on on the southwest border."

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DHS Secretary Alejandro Mayorkas on Tuesday warned that the U.S. is on track to encounter more migrants than in the last 20 years, calling it a "difficult" situation.

Mayorkas will testify to the House Homeland Security Committee on Wednesday, while Republican Sens. Ted Cruz and John Cornyn are leading a delegation to the border next week.