Rio security
Brazilian soldiers in riot gear participate in a recent ceremony to present the security forces lined up for the Rio Olympics. Another 1,000 US spies will be watching out for American athletes. Evaristo Sa/ AFP

As many as 1,000 spies from US intelligence and security agencies are reportedly keeping a watchful eye on Yank athletes and support staff at the Olympics.The massive surveillance team includes analysts, law enforcement and special operations personnel drawn from all 17 US intelligence agencies, including those in the military.

According to an NBC News review of a classified report on protection for the American Olympic crew, there are hundreds of security personnel on the ground in Rio, with hundreds more back in the states relying on eyes in the sky and on the Internet, taking particular care to track Isis-linked web sites. They'll be backed up by spy satellites and cutting-edge electronic eavesdropping equipment.

In addition, more than a dozen Navy and Marine Corps commandos from the US Special Operations Command are working in Brazil with federal police there and Brazil's Navy, military officials told NBC.

"US intelligence agencies are working closely with Brazilian intelligence officials to support their efforts to identify and disrupt potential threats to the Olympic Games in Rio," said Richard Kolko, a spokesman for National Intelligence Director James Clapper.

The major concern: a terrorist attack, although there is no known indication of a specific threat. But US military units are poised to move into action in the event of a needed rescue or counter-terrorism operation.

In July, authorities arrested a dozen Brazilian residents for suspected links to Isis. Brazil's justice minister described the men as "amateurs," but did note that they had discussed attacking the Olympics.

The central government pledged to release an extra $24m (£18m) to shore up safety measures about the same time. Up to 85,000 police and soldiers are set to be on patrol during the Olympics.

According to the intelligence review seen by NBC, US officials began planning its Olympic Watch operation late in 2015. The effort involved all federal intelligence and security agencies, such as the CIA, the National Security Agency, the FBI and the Secret Service. It also included the National Reconnaissance Office, which is responsible for operating spy satellites, and the National Geospatial Intelligence Agency, which is in charge of interpreting images spotted via satellite.

The NSA took the lead in the operation as it did in for the 1984 Olympics Los Angeles games.

The US is one of 51 nations supplying intelligence to the Brazilian counter-terrorism safety measures at the Olympics, but the size and sophistication of the American operation is second only to Brazil's.