Hezbollah on Sunday fired missiles from Lebanon at the town of Qosayr in response to the killing of 75 of the Shiite group’s fighters in Syria, Syrian opposition sources reported on Monday.

Syrian rebels ambushed and killed 75 Hezbollah militiamen who entered the Homs suburb of Qosayr, the British-based Strategic Research and Communication Center said. Other Syrian sources reported that the Hezbollah fighters came to assist a Syrian army assault on rebel positions in the Homs suburb on Sunday.

The Lebanese Shiite group responded by shelling the town with rockets. Syrian rebels reported that they still thwarted the government advance.

Monday’s report of Hezbollah involvement in the Syrian civil war followed an Al Arabiya report published on Friday which claimed that leaked documents showed that “Hezbollah has played an active role in the deadly crackdown on civilians and armed opposition in the neighboring Syria.”

According to the report, 250 Hezbollah fighters were dispatched to Aleppo in May 2011 to assist President Bashar Assad in quelling unrest. Other documents claimed to show Hezbollah’s involvement in the Syrian civil war since.

Last week, Hezbollah reported that one of its commanders was killed while “performing his jihadi duties” in Syria. It was not immediately clear how the Hezbollah militants were killed or whether they had been fighting alongside the Syrian army. It did not say when or where he was killed.

Lebanese opposition MPs criticized Hezbollah’s involvement in the Syrian civil war and its insistence on keeping a sizable arsenal, the Lebanese Daily Star reported on Monday. The paper wrote that Lebanese Forces party MP Antoine Zahra said Hezbollah could no longer claim its weapons were intended for fighting Israel in light of reports of direct involvement in the Syrian civil war.

“The resistance can no longer claim that its arms are to defend Lebanon because it is being used locally and against the Syrian people as well as in an attempt to gain control over Lebanon,” Lebanese newspaper Al-Joumhouria quoted him saying.

UN Secretary-General Ban Ki-moon on Monday expressed concern that the crisis in Syria could spill over to Turkey and Lebanon.