The officials, who spoke to Reuters on condition of anonymity, declined to say who was suspected of the attack but Washington is on heightened alert for activity by Iran-backed groups amid soaring tension in the region over the Israel-Hamas war.

Last week, Iraqi armed groups aligned with Iran threatened to target U.S. interests with missiles and drones if Washington intervened to support Israel's conflict with Hamas in Gaza.

The Pentagon has rushed air defenses and munitions to Israel, America's closest ally in the Middle East, but U.S. forces have not joined the fighting.

The one-way attack drones were intercepted as they attempted to strike Iraq's al Asad air base, which hosts American troops, the officials said.

The attack came hours after an explosion at a Gaza hospital killed hundreds of Palestinians, raising the stakes for U.S. President Joe Biden as he flies to Israel on Wednesday to signal support for its war against Hamas.

Israel blamed the blast at Al-Ahli al-Arabi hospital on a failed rocket launch by the Palestinian Islamic Jihad group, which denied responsibility.

Palestinian officials said an Israeli air strike hit the hospital, with the Palestinian Authority's health minister accusing Israel of causing a "massacre".

In Iraq, tension over the war in Gaza had already been high. Its top Shi'ite cleric, Grand Ayatollah Ali al-Sistani, last week condemned Israel and called on the world to stand up to the "terrible brutality" in Gaza.

Leaders of Iraqi armed groups blamed Israel for the attack on the hospital. Some of them condemned the U.S. for supporting Israel.

Kataib Hezbollah, a powerful armed faction with close ties to Iran, accused the United States of supporting Israel in "killing innocent people" and said it should leave Iraq.

"These evil people must leave the country, otherwise they will taste the fire of hell in this world before the afterlife," the group said in a statement late on Tuesday.

Iraqi politician Hadi Al-Amiri, leader of the political and military group the Badr Organisation which is close to Iran, also blamed Israel for the attack on the hospital and described it as "the massacre of the era, which can only be classified as a war crime" and condemned the U.S. and Western countries for supporting Israel.

We "will not hesitate to consider America and the West as partners in this hideous massacre", he said in a statement on Tuesday night. Last week he threatened to target U.S. interests if Washington intervened to support Israel.

The United States currently has 2,500 troops in Iraq - and an additional 900 in neighbouring Syria - on a mission to advise and assist local forces in combating Islamic State, which in 2014 seized swathes of territory in both countries.

In past years, Iranian-backed militias in Iraq regularly targeted U.S. forces in Iraq and the U.S. embassy in Baghdad with rockets, though such attacks have abated under a truce in place since last year, as Iraq enjoys a period of relative calm.

U.S. officials have accused Kataib Hezbollah of previous attacks on U.S. interests. The group has denied the claims.

Dozens of members of the Popular Mobilisation Forces (PMF), the state paramilitary organisation that contains many Iran-backed factions, took to the streets on Tuesday to condemn the hospital attack.

Demonstrators chanted anti-U.S. and anti-Israeli slogans and said they wanted to storm the U.S. embassy for its support of Israel.

A Reuters witness said that some of the protesters tried to cross the bridge that leads to the fortified Green Zone - home to the U.S. embassy and other missions in Baghdad - but were blocked by security forces.

(Reporting by Phil Stewart in Tel Aviv, Amina Ismail in Erbil, Ahmed Rasheed and Ahmed Saad in Baghdad; Editing by Clarence Fernandez, Nick Macfie and Bernadette Baum)

By Phil Stewart and Amina Ismail