US President Joe Biden promised "ironclad" support for Israel against attacks by Iran on Saturday, with American forces shooting down Iranian drones amid a spiraling crisis in the Middle East.

Biden cut short a weekend trip to the Delaware coast and held an emergency meeting at the White House with his top national security officials after Iran launched its unprecedented attack.

"I just met with my national security team for an update on Iran's attacks against Israel. Our commitment to Israel's security against threats from Iran and its proxies is ironclad," Biden said on X.

He posted a picture of the meeting in the wood-paneled White House Situation Room with officials including Defense Secretary Lloyd Austin, Secretary of State Antony Blinken and CIA Director Bill Burns.

Iran had vowed retaliation after a presumed Israeli strike on April 1 leveled an Iranian diplomatic building in Damascus, killing seven members of the elite Revolutionary Guards including two generals.

After launching the drone strikes, Tehran warned the United States Sunday to "stay away" from its conflict with Israel.

But US forces later shot down some of the Iranian drones, in a further show of support for the key regional ally it has supplied with billions of dollars in military aid for its six-month-old war in Gaza.

"US forces in the region continue to shoot down Iranian-launched drones targeting Israel," a US defense official said.

"Our forces remain postured to provide additional defensive support and to protect US forces operating in the region."

- 'Threats' -

US National Security Council spokeswoman Adrienne Watson said separately that the United States would "stand with the people of Israel and support their defense against these threats from Iran."

Biden was in "constant communication" with the Israelis and other allies, Watson added.

The US president, wearing a blue baseball cap, had earlier made no comment to waiting reporters as he boarded his helicopter Marine One in Rehoboth Beach, Delaware, to fly back to Washington.

He had been due to stay in the coastal town where he owns a house until Sunday, but cut the trip short for consultations on the Middle East situation, the White House said.

Biden had arrived in Rehoboth less than 24 hours earlier, shortly after warning Iran not to attack Israel but saying he expected an attack sooner rather than later.

Biden's handling of the Middle East conflict will also be under scrutiny in a US presidential election year.

Former US president Donald Trump, Biden's rival in November's election, said the Democratic incumbent was showing "weakness."

"God bless the people of Israel. They are under attack right now. That's because we show great weakness," Republican Trump said at a campaign rally in Pennsylvania.

Earlier Saturday, White House National Security Advisor Jake Sullivan and defense chief Austin both spoke to their Israeli counterparts to assure them of US support.

Tensions had ratcheted up earlier in the day when Iran's Revolutionary Guards seized a container ship near the Strait of Hormuz that was "related to the Zionist regime," the term it uses for Israel, state media reported.

The White House condemned the seizure of the British-owned vessel as an "act of piracy."

"We call on Iran to release the vessel and its international crew immediately," Watson said.

On Thursday the Pentagon said the top US commander for the Middle East had traveled to Israel for talks on security threats with the country's military officials.

The Strait of Hormuz connects the Gulf with the Indian Ocean and, according to the US Energy Information Administration, more than a fifth of global oil consumption passes through it each year.

dk-wd/des