"Twelve pro-Iranian fighters were killed in air strikes of unknown origin targeting their positions in the city of Deir Ezzor and to the east of the city, as well as the Boukamal region, near the border with Iraq," said the Syrian Observatory for Human Rights.

The strikes were not immediately claimed by any entity, according to the monitor.

Five of the strikes had targeted military positions near Deir Ezzor airport, it added.

Iran has been providing military aid to Syria since the civil war there began in 2011, while Israel has carried out hundreds of strikes targeting pro-Iranian groups in eastern Syria. The United States has also targeted such groups in the country's east.

Israeli authorities rarely comment on individual strikes in Syria, but have repeatedly said they will not allow arch-enemy Iran to expand its presence there.

Israel has launched an intense bombing campaign against Hezbollah strongholds in Lebanon in recent days, intensifying fears of a regional war.

The Israeli army has also repeatedly targeted the movement's arms supply routes on the Syrian-Lebanese border, according to the Syrian Observatory for Human Rights.