A complaint was lodged with the United Nations. The army is checking whether fire had indeed been directed at Israel.
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Meanwhile, Syrian activists reported that about 30 Hezbollah fighters and 20 Syrian soldiers and militiamen loyal to President Bashar Assad were killed in heavy fighting with rebels on Sunday in the town of Qusair.
According to Al-Arabiya TV, their bodies were transported to hospitals in Beirut. One of those killed was senior Hezbollah official Fadi al-Jazar.
According to the report, al-Jazar had previously been held in Israel and released as part of a prisoner exchange deal. In addition, 62 wounded, many of them Hezbollah combatants, were taken to hospitals in Lebanon.
Rebels in Qusair (Photo: AP)
Syria's state news agency SANA said the army had "restored security and stability to most Qusair neighborhoods" during the offensive in the town, about 10 km (six miles) from the Lebanese border.
On Sunday, Syrian troops aided by Hezbollah launched a massive assault on the rebel-held city near the Lebanese border.
Speaking from Qusair, activist Hadi Abdallah said Syrian warplanes bombed Qusair in the morning and shells were hitting the town at a rate of up to 50 a minute. At least 52 people were killed, he said.
"The army is hitting Qusair with tanks and artillery from the north and east while Hezbollah is firing mortar rounds and multiple rocket launchers from the south and west," he said.
"Most of the dead are civilians killed by the shelling."
The region near the Orontos River has been segregated into Sunni and Shiite villages in the civil war that grew out of protests against Syrian President Bashar Assad.
It is vital for Assad, who belongs to the Alawite sect, an offshoot of Shiite Islam, to keep open a route from Shiite Hezbollah's strongholds in the Bekaa to areas near Syria's Mediterranean coast inhabited by co-religionist Alawites.
Maor Buchnik and Reuters contributed to this report
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