Britain's Keir Starmer assured Israel's Prime Minister Benjamin Netanyahu on Tuesday of his country's "steadfast commitment" to Israeli security and protection of civilians after Iran's missile attack.

A spokesperson for Starmer's office said the prime minister spoke with Netanyahu on Tuesday afternoon, and the leaders discussed the escalating situation across the Middle East.

Starmer condemned Iran's attack on Israel, which began during the leaders' conversation, in the "strongest terms", the spokesperson added.

"It cannot be tolerated," Starmer told reporters later on Tuesday. "We stand with Israel and we recognise her right to self defense in the face of this aggression. Iran must stop these attacks."

Iran fired a salvo of ballistic missiles at Israel on Tuesday in retaliation for Israel's campaign against Tehran's Hezbollah allies in Lebanon, and Israel vowed a "painful response" against its enemy.

Asked whether Britain was prepared to use its military to help Israel defend itself, Starmer said Israel has the right to self defence and "any relevant updates will be provided in due course".

In Washington, President Joe Biden said the United States was prepared to help Israel defend itself.

Alarms sounded across Israel and explosions could be heard in Jerusalem and the Jordan River valley, and Israelis piled into bomb shelters. Reuters journalists saw missiles intercepted in the airspace of neighbouring Jordan.

Starmer spoke with Jordan's King Abdullah, and they underscored the urgent need for a ceasefire in both Lebanon and Gaza, Starmer's office said.

Starmer said he also held calls with French President Emmanuel Macron and German Chancellor Olaf Scholz to "try to find the space for a political solution".

A Downing Street spokesperson said the leaders acknowledged "the serious risk of regional escalation" and agreed all sides should show restraint.

On X, British Foreign Minister David Lammy said he "warned Iran against taking action that could push the region further towards the brink. A cycle of escalation is in no one's interest."