Approximately seven mortar rounds landed in the U.S. Embassy compound in Baghdad during an attack early on Friday, a U.S. military official told Reuters, in what appeared to be the largest attack of its kind in recent memory.

U.S. forces in Iraq and Syria were also targeted with rockets and drones at least five more times on Friday; three times at separate bases in Syria, and twice at the Ain al-Asad airbase west of Baghdad, a different U.S. defense official said.

The attacks were the most recorded against U.S. forces in the region in a single day since mid-October, when Iran-aligned militias started targeting U.S. assets in Iraq and Syria over Washington's backing of Israel in its war against Hamas in Gaza.

U.S. Defense Secretary Lloyd Austin, in a call with Iraq's Prime Minister Mohammed Shia Al-Sudani, condemned the attacks and singled out Iran-aligned armed groups Kataib Hezbollah and Harakat Hezbollah al Nujaba for the recent targetting of U.S. personnel.

"The United States reserves the right to respond decisively against those groups," Austin told Sudani, according to a Pentagon statement summarizing the call.

The embassy attack marked the first time it had been fired on in more than a year, apparently widening the range of targets. Dozens of military bases housing U.S. forces have been attacked, increasing fears of a broadening regional conflict.

No group claimed responsibility, but previous attacks against U.S. forces have been carried out by Iran-aligned militias operating under the banner of the Islamic Resistance in Iraq.

The U.S. military official, speaking on the condition of anonymity, left open the possibility that more projectiles were fired at the embassy compound but did not land within it.

The U.S. officials said Friday's attacks caused no injuries, and the embassy attack caused very minor damage.

Reuters was first to report the number of mortars that hit the embassy compound.

Explosions were heard near the embassy, in the centre of Baghdad, at about 4 a.m. (0100 GMT) on Friday. Sirens calling on people to take cover were activated.

State media said the attack damaged the headquarters of an Iraqi security agency.

Sheikh Ali Damoush, a senior official in the Lebanese group Hezbollah, said in a Friday sermon that attacks by Iran-aligned groups across the Middle East aim to apply pressure for a halt to Israel's offensive in the Gaza Strip. He did not refer specifically to Friday's attack.

U.S. forces in Iraq and Syria have been attacked at least 84 times since Oct. 17, the defense official said.

The U.S. has responded with a series of strikes that have killed at least 15 militants in Iraq and up to seven in Syria.

The State Department called on the Iraqi security forces to immediately investigate and arrest the perpetrators.

"The many Iran-aligned militias that operate freely in Iraq threaten the security and stability of Iraq, our personnel, and our partners in the region," State Department spokesperson Matthew Miller said in a statement.

"The Iraqi government has repeatedly committed to protect diplomatic missions as well as U.S. military personnel, who are present in the country at Iraq’s invitation. This is non-negotiable, as is our right to self-defense," Miller added.

'ACTS OF TERRORISM'

The attacks pose a challenge for Sudani, who has pledged to protect foreign missions and capitalize on fragile stability to focus on the economy and court foreign investment, including from the United States.

Sudani directed security agencies to pursue the perpetrators, describing them as "unruly, lawless groups that do not in any way represent the will of the Iraqi people," a statement from his office said.

He also said that undermining Iraq's stability, reputation and targeting places Iraq has committed to protect were acts of terrorism.

The head of militia Kataib Sayyid al-Shuhada, one of the main factions that has been targeting U.S. forces in the region, said in a social media post that he rejected "stopping or easing operations" while "Zionist crimes continue in Gaza."

Aside from its diplomatic staff in Iraq, the United States has about 2,500 troops in the country on a mission it says aims to advise and assist local forces trying to prevent a resurgence of Islamic State, which in 2014 seized large swathes of both Iraq and Syria before being defeated.

Iran-aligned Houthis in Yemen have been firing at Israel and ships in the Red Sea in a campaign they say aims to support the Palestinians. U.S. warships have shot down several of their projectiles.