The Department of Homeland Security has identified over 400 individuals brought to the U.S. from Eastern European and Central Asian countries by an ISIS-affiliated smuggling network in the past several months, a DHS senior official confirmed to Fox News on Wednesday.

Officials have arrested over 150 so far that have either been removed, placed in removal proceedings, or are currently receiving additional screening. They would not confirm if or how many have matched the terror watchlist. Of the remaining 50 individuals at large, they acknowledged that a small number "may match the terrorism watch list."

"There is no intelligence that suggests the remaining individuals pose an imminent threat to the homeland," the official said.

ICE SOUNDS ALARM ON WHAT MIGRANTS ARE FAILING TO BRING TO BORDER AS FEARS MOUNT AFTER ISIS ARRESTS

The officials said the majority of the 400 are not considered "subjects of concern" – and that only a small number are considered that. The network was identified because one of the organizers was determined to have ties to the Islamic terror group, and so, out of precaution, they wanted to identify, apprehend and screen everyone connected to the network.

At a press conference in Arizona on Wednesday, Homeland Security Secretary Alejandro Mayorkas emphasized that it was "incorrect" to say that 400 migrants with potential ties to ISIS had been brought to the U.S.

"We do not have identified 400 people with potential ISIS ties, and let me again assure you that individuals who are identified to have those ties would pose a concern to us from a public safety and security perspective and would be priorities for detention and removal," he said. "Precision when one is dealing with the facts is very, very important."

The details of the network were first reported by NBC News. The latest revelations come weeks after eight Tajikistan nationals with ties to ISIS were busted by federal authorities in New York City, Los Angeles and Philadelphia. All eight came in via the southern border illegally, but no derogatory information was flagged during processing. 

DHS does not believe this is an ISIS terror operation, but a network taking advantage of migrants seeking entry to the U.S. However, there is heightened security risk due to their countries of origin. Border officials are being instructed to detain any individuals coming from certain countries or associated with the network so additional vetting can be carried out.

NBC reported that recent terror attacks in Russia by ISIS and its offshoots have raised concerns and that DHS has been looking closely at migrants from Central Asia. 

While authorities say migrants are fully vetted before being released into the U.S., officials have previously conceded that it is difficult to vet them when often they turn up with no information. Acting Immigration and Customs Enforcement (ICE) Director Patrick Lechleitner said recently that "sometimes there is just no information on individuals" when they’re being vetted. 

"It is quite common where there is just nothing, you don't have anything. There's no criminal convictions, no threat information or whatever on these individuals. Or maybe these individuals are from an area that is particularly of concern but that pops up later when you get information later on," he said. 

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That is also within the context of broader concerns about the terrorist threat facing the U.S. A letter sent Tuesday from the Special Operations Association of America (SOAA), which represents thousands of Army Rangers, Green Berets, Navy SEALs, and other current and former elite special ops forces, warned its members are "gravely concerned" about the terrorist threat to the United States and Americans abroad.

"Our complete withdrawal from Afghanistan, without a viable stay-behind or over-the-horizon counterterrorism and intelligence capability to suppress threats, has created a vacuum in the region in which anti-American terrorist groups, such as al-Qaeda and the Islamic State, are flourishing once again," the letter states. 

The lack of intelligence collection in the region has put the United States "at risk."

Compounding the terrorist threat is the "unsecured southern border," according to the letter. "We do not know how many other terrorists are already currently inside the U.S."

CLICK HERE FOR MORE COVERAGE OF THE BORDER SECURITY CRISIS

The letter from former national security officials is the latest chorus of warnings about the terrorist threat.

"There's a lack of a sense of urgency here," former CIA acting Director Michael Morell told CBS' "Face The Nation" Sunday. "The American people need to understand what the threat is."

Republicans immediately responded to the revelations of the smuggling network by blaming the Biden administration and its policies at the southern border.

"I am a proud former U.S. Air Force fighter pilot who fought ISIS in Syria and Iraq. Now the Biden administration has opened our border, allowing these terrorists to smuggle illegal aliens into our country," Rep. August Pfluger, R-Texas, chairman of the House Homeland Security Subcommittee on Counterterrorism, told Fox News Digital. "I will seek immediate action from the administration and work with my colleagues to address any security gaps. The safety of our nation must always be our top priority."

Fox News’ Lucas Tomlinson and Bill Melugin contributed to this report.