Intelligence officials have determined that Islamic extremists have explored using the refugee program to enter the United States, they told the head of the Homeland Security Committee.

Rep. Michael McCaul (R-Texas) revealed portions of a classified letter from the National Counterterrorism Center (NCTC) on Monday, which offered new claims not previously disclosed by the Obama administration.

The disclosure could give ammunition to critics of the White House’s refugee plans who have warned that the program is vulnerable to infiltration by adherents of the Islamic State in Iraq and Syria (ISIS).

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The NCTC has identified “individuals with ties to terrorist groups in Syria attempting to gain entry to the U.S. through the U.S. refugee program,” the intelligence agency told McCaul in a letter.

“The refugee system, like all immigration programs, is vulnerable to exploitation from extremist groups seeking to send operatives to the West,” the agency added, noting that a small number of Iraqi refugees were arrested on terror charges in 2010.

McCaul disclosed the NCTC’s analysis during a speech at the National Defense University. He initially claimed that ISIS, in particular, had considered taking advantage of the refugee program, though his office later called that a “misstatement.”

“That was very courageous for them to come forward with this, to tell me about this personally, given the political debate on the Hill,” McCaul said on Monday.

The White House would not confirm or deny McCaul’s claim, which he was initially reluctant to link to the NCTC.

Press secretary Josh Earnest defended the U.S. refugee screening process, which typically takes two years for an individual to complete.

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“Those individuals who seek to enter the United States through the refugee resettlement program do so only after undergoing the most intensive screening of anybody who attempts to enter the United States," Earnest reporters on Monday.

A spokesman with the Office of the Director of National Intelligence also declined to comment on the record about the assertion.

McCaul’s speech comes amid escalating concerns about Islamic extremism in the West, following the killings of 130 people in Paris and 14 in San Bernardino, Calif. The massacres have caused American fears about terrorism to spike, and left Congress grasping for a response.

President Obama has repeatedly pushed back against warnings that ISIS could attack the United States by sending over fighters as refugees.

The Syrian civil war has killed more than 200,000 people since it began in 2011, and forced more than 4 million refugees to flee.

Still, Republicans and some Democrats have been wary of the president’s plans to allow 10,000 Syrian refugees into the country next year.

Last month, the House voted overwhelmingly to make it more difficult for refugees from Iraq and Syria to enter the United States.

In his speech Monday, McCaul painted a bleak picture of American national security, which he compared to the World War II fight against fascism.

The current threat posed by Islamic extremists is greater than at any time since 9/11, he said, and ISIS is “now more dangerous than al Qaeda ever was under Osama bin Laden.”

“I believe the state of our homeland is increasingly not secure,” McCaul said.

“I believe 2015 will be seen as a watershed year in this long war — the year when our enemies gained an upper hand and when the spread of terror once again awoke the West.”

The chairman's dire outlook stood in contrast to the message from Obama, who less than 24 hours before attempted to reassure the nation about terrorism in his third-ever address from the Oval Office.

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“The threat from terrorism is real, but we will overcome it,” Obama said.

“We will destroy ISIL and any other organization that tries to harm us. Our success won’t depend on tough talk, or abandoning our values, or giving into fear,” he added, using an alternate acronym for ISIS. “Instead, we will prevail by being strong and smart, resilient and relentless, and by drawing upon every aspect of American power.”

McCaul's comments could signal a renewed commitment to keep national security at the forefront of Congress’s focus, even as it rushes to finish its legislative work and adjourn for the year.

This week, the House is expected to take up legislation tightening a program allowing travelers from 38 countries to enter the U.S. without a visa. The reform is the only White House recommendation that Congress is likely to act on in the wake of the recent killings.

“This is just the beginning,” McCaul promised. “Over the coming weeks, we will introduce a slate of new bills based on the findings of the [bipartisan congressional] task force to keep terrorists from crossing our borders.”

Among other steps, McCaul said that lawmakers were interested in further hardening the country’s borders against extremists, sharing intelligence with foreign countries and developing a special commission to examine challenges posed by the proliferation of encryption technologies, which prevent the government from intercepting a suspect’s messages.

This story was updated at 5:35 p.m. to reflect McCaul's latest comments.

Jordan Fabian contributed to this story.