Iran's threats of reprisals against Israel after a strike in Syria this month that killed two Iranian generals remain "real," the White House said on Friday.

"We still deem the potential threat by Iran here to be real, to be viable," National Security Council spokesman John Kirby told journalists.

Kirby would not confirm reports that US officials considered an Iranian attack "imminent" or comment on whether the United States would help shoot down any Iranian missiles fired at Israel.

"I would just say that we're watching this very, very closely," he said, adding that Washington would make sure the Israelis "have what they need and that they're able to defend themselves."

The United States would also look at its own force posture in the region where it has thousands of troops "to make sure that we're we're properly prepared."

US President Joe Biden on Wednesday warned that Iran is "threatening to launch a significant attack on Israel." He pledged "ironclad" support for Washington's top regional ally despite diplomatic tensions over Israel's military conduct in Gaza. (AFP)