Beirut, Lebanon, April 14, 2018 (AFP) -

An explosion rocked an area in northern Syria Saturday near a base where pro-regime Iranian fighters and allied Shiite militias have been stationed, a monitor said.

The origin of the blast was not immediately clear and could have been caused by an air strike or an incident at an ammunition depot, said the Syrian Observatory for Human Rights.

"A powerful explosion went off late Saturday in an area of southern Aleppo province," the Britain-based monitoring group said. The war monitor's head, Rami Abdel Rahman, said the area was known to host Iranian forces deployed in Syria to support President Bashar al-Assad's regime as well as allied Afghan fighters from the Fatemiyoun Brigade.

Western powers carried out their biggest attack on Assad's regime before dawn on Saturday with strikes on targets they said were linked to Syria's chemical weapons program.

The Pentagon subsequently said no further strikes were planned as part of an operation launched a week after a suspected chemical attack on the holdout rebel town of Douma killed dozens of people.

Israel, which was not involved in Saturday's unprecedented wave of missile strikes but expressed full support, has unilaterally launched air raids on targets inside Syria on several occasions.