Multiple B-2 bomber aircraft left a base in the central United States overnight and were later tracked flying off the California coast along with aerial refueling jets, The New York Times and specialist plane tracking sites reported.

The B-2 is capable of carrying America's heaviest payloads, including the bunker-busting GBU-57, a 30,000-pound (13,607 kg) warhead capable of penetrating 200 feet (61 meters) underground before exploding.

Such a bomb, which Israel is not known to possess, is the only weapon capable of destroying Iran's deeply buried nuclear facilities.

When reached for comment, the Pentagon referred AFP to the White House, which did not immediately respond.

Trump, who rarely spends weekends in Washington, is due to return to the White House on Saturday evening to hold an unspecified "National Security Meeting."

The president said Friday that Iran had a "maximum" of two weeks to avoid possible US air strikes, indicating he could take a decision before the fortnight deadline he had announced a day earlier.