The Washington Post said Caine, the chairman of the joint chiefs of staff, had expressed concern at the White House and Pentagon that munition shortages and a lack of allied support could increase danger to US personnel.

And the Wall Street Journal said both the top general and other Pentagon officials have warned of risks such as US and allied casualties as well as the danger of US air defenses being depleted if its forces strike Iran.

Trump -- who ordered strikes on Iran last year -- has repeatedly threatened Tehran with further military action if ongoing talks do not reach a replacement for the nuclear deal the US president tore up in 2018, during his first term in office.

Washington has deployed a massive military force to the Middle East, sending two aircraft carriers as well as more than a dozen other ships, a large number of warplanes and other assets to the region.

But negotiations are still set to continue, with a US official saying the next round of talks with Tehran would take place on Thursday.