US President Joe Biden has ordered the military to "assist Israeli defenses" and shoot down Iranian missiles targeting Israel, the White House said in a statement.

The statement said that Biden and his deputy, Kamala Harris, the Democratic presidential candidate, are following the Iranian attack on Israel from the White House operations room.

For its part, the Washington Post quoted a Pentagon official as saying that its forces are actively defending against missiles launched by Iran.

Israel announced this evening that Iran had fired about 250 missiles at it, in an attack that Tehran said was in retaliation for the assassination of both the Secretary-General of the Lebanese Hezbollah, Hassan Nasrallah, and the head of the political bureau of the Hamas movement, Ismail Haniyeh.

Israel assassinated Nasrallah in an airstrike on the southern suburbs of Beirut on September 27, while Hamas political bureau chief Ismail Haniyeh was assassinated in a bombing of his residence during a visit to Tehran at the end of July.

The New York Times quoted American officials as saying that Washington fears the development of an exchange of fire between Iran and Israel.

According to American officials, the most extreme scenarios of what is happening include Israel striking Iranian nuclear facilities.

The newspaper quoted these officials as saying that Washington had recently been assessing how the exchange of fire between Iran and Israel could develop.

In the context, UN Secretary-General Antonio Guterres denounced "the expansion of the conflict in the Middle East with one escalation after another" after Iran launched a barrage of ballistic missiles at Israel, adding in a statement on Tuesday, "This must stop. We absolutely need a ceasefire."