Washington DC - US stealth bombers were flying Saturday across the Pacific Ocean, according to tracking data and media reports, fueling speculation over their intended mission as President Donald Trump considers joining Israel's attack on Iranian nuclear sites.

Multiple B-2 bomber aircraft left a base in the central US 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 warhead capable of penetrating 200 feet 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.

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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.

Iran's President Masoud Pezeshkian said Saturday his country will not halt nuclear activity "under any circumstances."

"We are ready to discuss and cooperate to build confidence in the field of peaceful nuclear activities, however, we do not agree to reduce nuclear activities to zero under any circumstances," said Pezeshkian during a phone call with French President Emmanuel Macron, according to the official IRNA news agency.