The FBI has dramatically slowed the pace of security reviews for refugees in recent months, which former Trump administration officials and human rights advocates say is part of an intentional bid by White House hardliners to restrict the number of refugees allowed in the U.S.

Former officials and aid organizations say the administration has overloaded the FBI and other government agencies with an array of procedures that have weighed down the bureaucracy and effectively delayed refugee admissions.

"It’s a precipitous decline," said one former official who worked on refugee issues before leaving government earlier this year.

Refugee admissions have plunged to historic lows. The U.S. is on track to admit only about 20,000 - 21,000 refugees by the end of September, far below a ceiling set at 45,000 by administration officials last year. That cap itself was lower than any set by a U.S. president since the current refugee program was created in 1980.

The delays in security screening coincide with an intense debate inside the administration over how many refugees should be permitted to enter the country next fiscal year, with political appointees pushing for an unprecedented brake on refugee admissions.

The FBI screening mainly involves sifting through databases to check if the refugee applicant has no links to extremists. On some days, the FBI has managed to review only a handful of cases, creating a bottleneck in the processing of refugee applications, humanitarian workers and two former officials who served under President Donald Trump told NBC News.

Another former official, who spoke on condition of anonymity, said the FBI’s slow pace was the main cause for the sharp drop in refugee admissions.

In a statement, the FBI said its primary focus is "to protect the United States from national security and criminal threats."

"We are not able to release the number of applications we have reviewed, but in the course of carrying out our duties the FBI takes the necessary time to thoroughly review all the information available. The FBI will continue to support the comprehensive and rigorous vetting of individuals who may be admitted to our country."

Administration officials insisted the FBI is not dragging its feet. The bureau is merely following new procedures that require checks of additional databases to weed out any potential terrorists, they said.

The FBI is one of several agencies involved in vetting refugees seeking to come to the United States, starting with the United Nations High Commissioner for Refugees and including the State Department and the Department of Homeland Security.

Apart from the lengthy screening process, DHS has also drastically cut the number of staff that manage refugee admissions and travel overseas to interview those seeking to resettle in the U.S.