Two rockets hit Hezbollah's heartland in Beirut on Sunday as the Lebanese Shiite group battled alongside Syrian regime forces and Damascus said it agreed "in principle" to attend a Geneva peace conference.
The early morning attack came as Syria's fractured opposition held an unscheduled fourth day of meetings in Istanbul on the peace conference proposal and after Hezbollah pledged to fight for "victory" in Syria.
Its chief Hassan Nasrallah said it was in the militant anti-Israeli group's own interest to defend President Bashar al-Assad's regime.
"I have always promised you a victory and now I pledge to you a new one" in Syria, he said.
Hours later, two Grad rockets slammed into Al-Shayyah area of southern Beirut, a security source said, wounding four Syrian workers at a car dealership.
It was the first time the Lebanese capital's mainly Shiite southern suburbs have been targeted during the more than two-year-old conflict in Syria.
An AFP photographer said the second rocket damaged an apartment block.
There was no immediate claim of responsibility for the attack which Interior Minister Marwan Charbel denounced as "sabotage".
"We hope that what is happening in Syria will not spill over into Lebanon," he said.
During the past week, 31 people have been killed in clashes in Lebanon's northern port of Tripoli between supporters and opponents of Syria's regime.
The army found two abandoned rocket launchers in Aitat, southeast of Beirut's southern suburbs, the security source said.
"The people will not be intimidated by such acts and are determined to defend the resistance (Hezbollah)... We will prevent all sectarian dissent," Hezbollah MP Ali Ammar said.
With the violence in Syria spreading, Syrian Foreign Minister Walid Muallem said his government would attend "in principle" the Geneva peace conference Washington and Moscow are hoping to hold next month.
"We think... that the international conference represents a good opportunity for a political solution to the crisis in Syria," Muallem said.
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The opposition's long-standing position is that, after more than two years of devastating conflict which has killed more than 94,000 people, it will not negotiate until Assad agrees to leave.
"We are ready to enter into negotiations that are aimed towards transferring power to the people, towards a democratic transition. And that of course means Assad cannot be a part of Syria in the future," opposition spokesman Louay Safi said in Istanbul.
Meeting in the Turkish city since Thursday, the deeply divided opposition National Coalition has yet to reach an official position on the peace conference dubbed Geneva 2.
Delegates said efforts to reach an agreed position were being delayed by pressure from some Gulf Arab backers for an overhaul of its membership.
"You have Saudi Arabia and the United Arab Emirates pushing to include up to 30 new members in the National Coalition," one said. "Their goal is to downsize the Muslim Brotherhood's influence over the group."
Syrian Kurds opposed to the regime also want to participate in the conference, either as Coalition members or not, representatives in Istanbul said.
French Foreign Minister Laurent Fabius said Paris would host on Monday a meeting with Russia and the United States to prepare for the conference, as China, an ally of Damascus, said it "is open and actively disposed" for a political solution.
Also on Monday, EU foreign ministers are to meet in Brussels with the bloc deeply divided over whether to arm rebels, ahead of the expiry at midnight Friday of sanctions against Syria, including a weapons embargo.
Hezbollah's intervention has given Assad the upper hand in Qusayr, a strategic central town which provides an important rebel supply line from Lebanon and serves as a link to Assad's Alawite heartland.
Syrian forces launched an assault on Qusayr a week ago but are still being fiercely resisted.
On Sunday a source close to Hezbollah said pro-Assad forces had taken control of 80 percent of Qusayr.
"We still have to take another 20 percent of Qusayr, since we took 10 percent on Sunday and the rest was already in our hands," the source said, declining to be named.
Even as Nasrallah vowed victory in Syria, 22 Hezbollah fighters were killed in the Qusayr battle on Saturday, the source said.
The Syrian Observatory for Human Rights said Hezbollah lost 10 fighters in Qusayr, out of a total of 147 people killed in violence across the country on Saturday, including 79 rebels.