More than 70 percent of the civilians working for US spy agencies have been deemed "non-essential" employees and face temporary layoffs due to the government shutdown that began on Tuesday, three officials familiar with the matter said.
The agencies affected are the Central Intelligence Agency, the Office of Director of National Intelligence and 15 others, the officials said.
The CIA expects to furlough about 12,500 civilians working for the agency, according to the sources. But specific numbers for other agencies were not immediately clear, the officials told Reuters. The CIA and White House declined to comment.
No uniformed military personnel will be affected, officials said. Some large spy agencies under control of the Pentagon employ as many, or more, military personnel than civilian workers.
These agencies include the Defense Intelligence Agency and the National Security Agency, whose secretive electronic eavesdropping methods recently become the focus of controversy following leaks by former contractor Edward Snowden.
The US government partially shut down for the first time in 17 years on Tuesday as a standoff between President Barack Obama and congressional Republicans over healthcare reforms closed museums and national parks and slowed everything from trade negotiations to medical research.
Federal agencies cut back services and up to 1 million workers were put on unpaid leave.
Shawn Turner, chief spokesman for the Director of National Intelligence, declined to discuss specifics of the layoffs but acknowledged that the performance of spy agencies could be affected if the shutdown continues.
"The Intelligence Community's ability to identify threats and provide information for a broad set of national security decisions will be diminished for the duration," Turner said.
"The immediate and significant reduction in employees on the job means that we will assume greater risk and our ability to support emerging intelligence requirements will be curtailed," he said.
"The fraction of Intelligence Community employees who remain on the job will be stretched to the limit and forced to focus only on the most critical security needs."
Some officials said that while spy agencies play critical roles in gathering information on foreign governments and potential terrorist threats, they are subject to the same budget laws as other government units. Under current circumstances, they must temporarily lay off large numbers of staff to comply with the law, the officials said.
Precisely how intelligence operations will be affected by the shutdown, and precise numbers of individuals affected, remain classified, officials said.
However, among the types of activities likely to be cut back as a result of the furloughs are strategic planning and analyses and some "foreign liaison" activities.
In many parts of the world, US agencies rely heavily on their partnerships with local spy outfits, such as Britain's MI5 and MI6 and Israel's Mossad.