A police cordon is seen blocking the access at the White House in Washington, DC, on 26 November 2019, during a lockdown started following an air space violation. (AFP)

The White House and US Congress were locked down for nearly an hour Tuesday over a suspected airspace intrusion over Washington, but hours later officials still had no explanation for what triggered the alert.

Major Andrew Hennessy, a spokesman for the North American Aerospace Defense Command, told AFP that “a track of interest” appeared on radars early Tuesday, sparking concern.

“Something that gave off a signature that our radar registered” set off the alert, he said.

A US Coast Guard helicopter deployed by NORAD to investigate what appeared to be an unidentified aircraft nearing tightly controlled Washington airspace found nothing.

“There was no aircraft to intercept,” Hennessy said.

Hennessy declined to comment on speculation that a flock of birds may have triggered the alert.

Hennessy also said an earlier NORAD tweet that said fighter jets had been scrambled to intercept the threat was incorrect.

The fighter jets reported were part of a previously scheduled exercise by the 113th Wing, District of Columbia Air National Guard unit based at Joint Base Andrews, just outside of Washington.

“It just happened to be at the same time. They were not involved in the interception,” said a spokesperson for Andrews.

The White House was locked down for the nearly an hour due to what the US Secret Service, which protects the president, described as “a potential violation of the restricted airspace in the National Capital Region.”

There was no information about what President Donald Trump and his wife Melania did during the alert.

The incident evoked memories of the 11 September 2001 attacks when Al Qaeda jihadists flew two airliners into the World Trade Center in New York and a third into the Pentagon.

A fourth airliner that crashed in a field in Pennsylvania was believed to have been intended for the White House or the Capitol.

Washington airspace restrictions were severely tightened after those attacks, which left nearly 3,000 dead.

And surface-to-air missile batteries have been installed around the city to target potentially attacking aircraft.

A number of Americans have flown aircraft into downtown Washington in the past.

In 1974, Robert Preston, a despondent army private who was 20 years old, landed a military helicopter on the White House lawn, after police cars tracked him on the ground and Secret Service agents fired at him with shotguns.

In April 2015, Florida mailman Doug Hughes, angry about how big money was allegedly corrupting US politics, flew undetected in a small gyrocopter into downtown Washington, landing it just outside the Capitol building.

He landed without incident, was arrested and a year later was sentenced to 120 days in jail for the felony crime of operating an aircraft without a pilot license.