US President Joe Biden has strongly urged Iran not to attack Israel in connection with an April 1 missile attack on the Iranian embassy in Syria.

Iran has blamed Israel for the fatal attack and vowed to retaliate.

At a news conference on Friday, Biden was asked how imminent he thinks an Iranian attack on Israel may be. He said his expectation is "sooner than later."

Asked about his message to Iran, Biden simply said, "Don't."

He added that the United States is devoted to the defense of Israel, and that Iran "will not succeed."

White House national security spokesperson John Kirby told reporters that the reportedly imminent Iranian attack on Israel is a real and viable threat. He said the US was looking at its own force posture in the region and watching the situation very closely.

Also on Friday, CBS News cited multiple officials as saying that a major Iranian attack on Israel may include more than 100 drones and dozens of missiles aimed at military targets inside the country.

The officials reportedly held out the possibility that the Iranians could opt for a smaller-scale attack to avoid a dramatic escalation.