Secretary of the Interior Doug Burgum issued a stark warning about a possible terrorist attack on the Statue of Liberty as the United States prepares to mark the 250th anniversary of its independence.

Speaking at a press conference in New York, Burgum praised the U.S. Park Police, the nation's oldest federal law enforcement agency, highlighting their crucial role throughout American history.

While discussing their work safeguarding the Statue of Liberty - amongst America's most recognisable landmarks - he observed it features prominently in imagery on the majority of terrorist recruitment websites, cautioning that it could one day become the target of an assault.

"The Statue of Liberty, by the way, is the most featured image on terrorist recruiting sites," Burgum said. "It's the thing that, if someone could wreck our 250th anniversary..."

He continued by explaining that the U.S. Park Police vets more than 4 million visitors to the monument annually before they embark on the renowned ferry crossing to the island where it stands.

"They protect the National Mall, the Golden Gate Recreation Area, but when you're here on this ground today, at least 26,000 acres, the law enforcement for this national recreation is not the Metro - it's not New York. It's the U.S Park Police," he said.

"They keep this facility safe. They keep the over 9 million visitors that visit this recreation area a year safe. They do a fantastic job."

In New York City, two men whom federal authorities claim were inspired by the Islamic State carried powerful homemade explosives to a far-right demonstration outside the mayoral residence.

These three violent incidents in March have exposed a heightened terrorism threat emerging against the backdrop of the U.S. conflict with Iran, at a time when the nation's counter-terrorism apparatus is under severe strain due to the departure of seasoned national security professionals from both the FBI and the Justice Department.

The sackings and resignations, coupled with the redirection of resources and personnel over the past year to fulfil other Trump administration priorities, have sparked serious concerns over the capacity to prevent a potential surge in threats.

"So much experience has been decimated from the ranks," said Frank Montoya, a retired senior FBI official.

"The folks that were best positioned to get to the bottom of it before something really bad happened" are in many cases no longer with the government, he said, meaning less experienced personnel assigned to the threat are "starting from way behind."

The FBI released a statement asserting that "agents and staff are dedicated professionals working around the clock to defend the homeland and crush violent crime. The FBI continuously assesses and realigns our resources to ensure the safety of the American people."

In Michigan, a naturalised citizen from Lebanon drove his vehicle into a synagogue, where he was shot at by security personnel before turning the gun on himself.