The Syrian Observatory for Human Rights monitor did not say who carried out the air strikes in the eastern province of Deir Ez Zor Ez Zor, which has been targeted by both US and Israeli raids in the past.

A US defence official told AFP the United States "did not conduct any airstrikes" overnight.

The Observatory said "an Iranian Revolutionary Guards adviser, his two Iranian security escorts", and four Syrian and nine Iraqi fighters with pro-Iran groups were killed in strikes on a villa in Deir Ez Zor city and on the town of Albu Kamal on the Iraqi border.

It said the villa "served as a communications centre" in the area, reporting that the building was destroyed and that a Syrian civilian was also killed.

WHO chief Tedros Adhanom Ghebreyesus announced on social media "the tragic loss of another one of our own in an air strike in Syria this morning".

The post identified him as Emad Shehab, an engineer who "served as the WHO water and sanitation focal point" in Deir Ez Zor.

Iran's state news agency IRNA reported the attack was "carried out by the Zionist regime", its term for Israel.

It identified the dead Guard as Behrouz Vahedi, saying he belonged to the Quds Force, the foreign operations arm of Iran's Islamic Revolutionary Guard Corps.

Syrian state news agency SANA blamed "American occupation forces" for the attack, which it said killed "seven military personnel and one civilian", wounded others and caused "great material losses to public and private property".

- 'Windows shattered' -

Israel has carried out hundreds of strikes targeting pro-Iran groups fighting alongside the forces of President Bashar al-Assad in the country's 13-year civil war.

The United States has carried out fewer strikes in eastern Syria against pro-Iran groups, which it blames for a flurry of attacks on US interests in Iraq and Syria during the Israel-Hamas war in Gaza.

Hammoud al-Jabbour, who lives a short walk from the villa, told AFP he was woken up by the sound of explosions.

"It was one of the biggest strikes I've heard," he said.

"The windows of my house were shattered, the power was cut in several neighbourhoods and the main roads were closed."

The Observatory said an Iranian cargo plane flew from Damascus to Deir Ez Zor city shortly before the strike, carrying technical equipment and the Guards adviser.

The Britain-based Observatory, which has a network of sources inside Syria, said he was in charge of telecommunications.

US strikes in February killed 29 pro-Iran fighters in the Deir Ez Zor and Mayadeen areas in response to a deadly drone attack on a US base that killed three US soldiers across the border in Jordan.

Pro-Iran groups have since cut back their attacks on US targets in Syria, according to the Observatory.

In early March, an Iranian Revolutionary Guard was killed along with two other people in an Israeli strike on the Syrian Mediterranean city of Banias.