U.S. President Joe Biden on Friday said he expected Iran to attack Israel "sooner, rather than later" and warned Tehran not to proceed.

Asked by reporters about his message to Iran, Biden said simply, "Don't," and he underscored Washington's commitment to defend Israel.

"We are devoted to the defence of Israel. We will support Israel. We will help defend Israel and Iran will not succeed," he said.

Biden said he would not divulge secure information, but said his expectation was that an attack could come "sooner, rather than later." He spoke to reporters at the White House after a virtual speech to a civil rights conference

Israel braced on Friday for an attack by Iran or its proxies as warnings grew of retaliation for an attack on Iran's embassy compound last week in Damascus that killed a senior commander in the Iranian Revolutionary Guard Corps' overseas Quds Force and six other officers.

Countries including India, France, Poland and Russia have warned their citizens against travel to the region, already on edge over the war in Gaza, now in its seventh month. Germany on Friday called on its citizens to leave Iran.

Israel did not claim responsibility for the airstrike on April 1. But Iran's supreme leader, Ayatollah Ali Khamenei, said Israel "must be punished and shall be" for an operation he said was equivalent to an attack on Iranian soil.

Earlier, White House spokesperson John Kirby said the reportedly imminent attack by Iran on Israel was a real and viable threat, but gave no details about any possible timing.

Kirby said the United States was looking at its own force posture in the region in light of Tehran's threat and was watching the situation very closely.