Israel is bracing for a direct attack by Iran as warnings grow of retaliation for the killing last week of a senior officer in Iran's embassy in Damascus. US and other intelligence assessments have said the retaliation could come as soon as Sunday. The unprecedented attack could trigger an all-out regional war.

US President Joe Biden has also warned Israel that he expects a strike from Iran soon, but has warned the clerical state not to attack.

"I don't want to get into secure information but my expectation is sooner than later," Biden told reporters after an event.

Asked what his message was to Iran on striking Israel, Biden said, "Don't."

An assault from Iranian soil has emerged as one of the main scenarios expected by the Jewish state and its allies, according to reports by the Wall Street Journal and Bloomberg. A bombardment with drones and precision missiles could come within the next 24 hours, the reports said citing people familiar with the matter.

Any Iranian attack on Israel would likely be a combination of missiles and drones, based on current capabilities outlined in a new Defense Intelligence Agency Worldwide Threat assessment released late Thursday.

The regime "has a substantial inventory of ballistic and cruise missiles capable of striking targets as far as 2,000 kilometers from its borders," the agency said.

The US has rushed additional military assets to protect Israel and American forces in the region. The country has moved two Navy destroyers to the Eastern Mediterranean Sea, according to a Navy official. One is the USS Carney, which was recently in the Red Sea performing air defence against Houthi drones and anti-ship missiles.

America has also doubled down its diplomatic efforts to rein in hostilities in the region, which has been on the edge since Israel launched a mega offensive on Palestine to destroy the militant organisation Hamas.

Israel has been on alert since then, canceling home leave for combat troops, calling up reserves, and bolstering air defenses. Its military scrambled navigational signals over Tel Aviv on Thursday to disrupt GPS-navigated drones or missiles that might be fired at the country.