Five militiamen from an Iranian-backed base in Syria were reportedly killed after what are widely believed to be Israeli strikes on Monday night.
Another eleven were wounded according to the British-based group, the British-based Syrian Observatory on Human Rights, while the Syrian news agency Sana reported that seven Syrian soldiers had been wounded.
The blast from one weapon depots, nine miles south of Damascus, was so loud that it reportedly rattled windows in the capital.
"There were several strikes and the blasts were huge," Al Jazeera quoted a Syrian analyst as saying.
Israel has rarely commented on its operations in Syria since the oubreak of the civil war nine years ago.
But Israeli leaders have pledged to prevent Iran from consolidating positions in the country which could be used to launch attacks on Israel.