Missile strikes on military sites in northern Syria overnight reportedly killed a number of pro-government fighters, including Iranians.
The Syrian military said facilities in Hama and Aleppo provinces were struck, reports BBC.
It did not say if there were any casualties. But a UK-based monitoring group said four Syrians and 22 foreigners, mostly Iranians, died.
It is not known who was behind the attacks. But Western nations and Israel have previously hit sites in Syria.
Earlier this month, the US, UK and France bombed three facilities they said were associated with the Syrian government's alleged chemical weapons programme.
Israel is meanwhile alleged to have hit an airbase reportedly serving as an Iranian drone command centre and containing an advanced Iranian air defence system.
The office of the Israeli Prime Minister Benjamin Netanyahu says he will issue a statement on Monday evening on a "significant development" regarding the nuclear deal between Western powers and Iran.
Mr Netanyahu has been a staunch opponent of the deal, arguing that it should be "fixed" or scrapped. US President Donald Trump is due to decide on 12 May whether or not to re-impose nuclear-related sanctions on Iran, a move which would put the agreement in jeopardy.