The United States said it was discussing a joint response after Iran fired a barrage of missiles at Israel, warning Tehran of "severe consequences".
Israel vowed it would make Iran "pay" after the attack late Tuesday, with most of the missiles intercepted, and pledged to immediately strike "the Middle East powerfully".
Tehran, in turn, threatened to strike infrastructure across Israel if its territory was attacked.
President Joe Biden said the United States was "fully supportive" of Israel after the missile attack, adding that he would discuss a response with Israeli Prime Minister Benjamin Netanyahu.
Asked by reporters what the response towards Iran would be, Biden replied: "That's in active discussion right now."
Palestinian youths pose for a picture atop a fallen projectile after Iran launched a barrage of missiles at Israel in response to the killings of Nasrallah and other Iran-backed militants, in Ramallah in the occupied West Bank. Photo: AFPMissiles shot down
Sirens sounded across Israel after Iran unleashed the missiles - most of which were intercepted by Israeli air defences or by allied air forces.
Iranian state media reported 200 missiles were fired at Israel including hypersonic weapons for the first time, which the Revolutionary Guards said had targeted "three military bases" around Tel Aviv and others elsewhere.
Iran's Foreign Minister Abbas Araghchi posted on social media platform X that Tehran's "action is concluded unless the Israeli regime decides to invite further retaliation".
The Revolutionary Guards earlier said the attack was in response to Israel's killing of Hezbollah leader Hassan Nasrallah last week as well as the death of Hamas leader Ismail Haniyeh in a Tehran bombing widely blamed on Israel.
Israeli medics reported two people lightly injured by shrapnel. In the occupied West Bank, a Palestinian was killed in Jericho "when pieces of a rocket fell from the sky and hit him", the city's governor Hussein Hamayel told AFP.
It was Iran's second direct attack on Israel after a missile and drone attack in April in response to a deadly Israeli air strike on the Iranian consulate in Damascus.
President Joe Biden said the United States was "fully supportive" of Israel after the missile attack, adding that he would discuss a response with Israeli Prime Minister Benjamin Netanyahu. Photo: AFP'Severe consequences'
US Defense Secretary Lloyd Austin slammed an "outrageous act of aggression" by Iran, while Biden's National Security Advisor Jake Sullivan told reporters there would be "severe consequences".
Netanyahu said "Iran made a big mistake tonight and will pay for it."
Iran reacted by threatening to fire "with bigger intensity" if its territory is attacked, with Major General Mohammad Bagheri warning Tehran would target "all infrastructure" in Israel.
Following the missile barrage, Israeli military spokesman Rear Admiral Daniel Hagari vowed the air force "will continue to strike (tonight) in the Middle East powerfully".
The military subsequently announced it was bombarding Hezbollah targets in Beirut, with a Lebanese security source telling AFP that Israel had hit the city's southern suburbs at least five times overnight.
UN chief Antonio Guterres led international calls to stem the "broadening conflict in the Middle East", saying in a statement: "This must stop. We absolutely need a ceasefire."
While Iran-backed groups across the region had already been drawn into the Gaza war, sparked by Palestinian group Hamas's October 7 attack on Israel, Tehran had largely refrained from direct attacks on its regional foe.
Iranian President Masoud Pezeshkian said his country had exercised its "legitimate rights" and dealt "a decisive response... to the Zionist regime's aggression".
Israel, Iraq and Jordan - which lie between Iran and Israel - closed their airspace, as did Lebanon before reopening.
US boosts forces
The escalation came after the Israeli military said early Tuesday that troops had started "targeted ground raids" in south Lebanon, across Israel's northern border.
The move came despite growing calls for de-escalation after a week of air strikes that killed hundreds in Lebanon.
Lebanon's health ministry said later that the latest Israeli strikes had killed a further 55 people on Tuesday.
Lebanon's disaster management agency said 1,873 people had been killed since Israel and Hezbollah began trading cross-border fire after the Gaza war erupted on October 7, 2023.
Iran has said Nasrallah's killing would bring about Israel's "destruction", though its foreign ministry said Monday that Tehran would not deploy any troops to confront Israel.
The Pentagon said Washington was boosting its forces in the Middle East by a "few thousand" troops.
Deadly strikes on Gaza
In Lebanon, the UN peacekeeping mission said the Israeli offensive did not amount to a "ground incursion" and Hezbollah denied that any troops had crossed the border.
There was no way to immediately verify the claims, which came as Israel struck south Beirut, Damascus and Gaza.
Israel says it seeks to dismantle Hezbollah's military capabilities and restore security to northern Israel, where tens of thousands have been displaced by nearly a year of cross-border fire.
Iran-backed Hezbollah, which suffered heavy losses in a spate of attacks last month, said it targeted Israeli military bases on Tuesday.
In Gaza, the civil defence agency said Israeli bombings killed 19 people on Tuesday.
The Israeli military said troops opened fire Tuesday on "dozens" of Palestinians in central Gaza they saw as an "immediate threat". At least some were hit, it added.
Hamas's October 7 attack on Israel resulted in the deaths of 1,205 people, mostly civilians, according to an AFP tally based on Israeli official figures that include hostages killed in captivity.
Israel's retaliatory military offensive has killed at least 41,638 people in Gaza, most of them civilians, according to figures provided by the Hamas-run territory's health ministry. The UN has described the figures as reliable.
'Lost my home'
Hezbollah began low-intensity strikes on Israeli troops a day after its Palestinian ally Hamas staged its unprecedented attack on Israel on October 7, which triggered Israel's devastating assault on Gaza.
The escalating violence in Lebanon has killed more than 1,000 people since September 17, Health Minister Firass Abiad said.
Lebanese Prime Minister Najib Mikati said there could be as many as one million people displaced from their homes in the country, with authorities registering almost 240,000 crossings into Syria since September 23.
In central Beirut, Youssef Amir, displaced from southern Lebanon, said: "I have lost my home and relatives in this war, but all of that is a sacrifice for Lebanon, for Hezbollah".
Beirut resident Elie Jabour, 27, told AFP that despite opposing Hezbollah "politically... I support them defending the border".