"Nine pro-Iranian fighters, including a Revolutionary Guards commander, were killed in air strikes targeting the villa they were staying in, which served as a communications centre," the Syrian Observatory for Human Rights said.

It said four people were killed in a separate strike in the town of Albu Kamal on the Iraqi border.

The Britain-based monitor said it had no word on who carried out the strikes and there was no immediate claim of responsibility.

Israel has carried out hundreds of strikes targeting pro-Iranian groups fighting alongside the forces of President Bashar al-Assad in the country's more than a decade-old civil war.

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

Media close to the Syrian government said the latest strikes were American.

The Observatory said that just one of the fighters killed in the villa was Syrian. It said that 10 Syrian civilians who lived nearby were among more than 30 people wounded.

"We heard loud explosions which woke us up, then the sound of ambulances," said Hammoud al-Jabbour, who lives less than 100 metres (yards) from the villa which was targeted.

"It was one of the biggest strikes I've heard -- the windows of my house were shattered, the power was cut in several neighbourhoods and the main roads were closed."

The Observatory said that a few hours earlier, an Iranian cargo plane flew from Damascus to the eastern city of Deir Ezzor carrying technical equipment and soldiers of Iran's Revolutionary Guards.

The villa which was targeted and completely destroyed in the strike had been taken over by the Guards, who are responsible for Iran's foreign operations.

The strikes were the first of their kind in eastern Syria since early February, the Observatory said.

Then, US strikes targeting the eastern cities of Deir Ezzor and Al-Madayeen killed 29 pro-Iran fighters in response to a deadly drone attack on a US base which killed three US soldiers just across the border into Jordan.

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

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