Two rockets slammed into the Hezbollah heartland of south Beirut on Sunday as fighters from the Lebanese Shiite group battled alongside regime forces against rebels in a key town in neighboring Syria.

The early morning attack came as Syria's fractured opposition began an unscheduled fourth day of meetings on a peace conference proposal and after Hezbollah pledged "victory" in Syria over the rebels.

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 in a speech for the 13th anniversary of Israel's withdrawal from Lebanon.

Hours after he spoke, two Grad rockets hit the Al-Shayyah area of south 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.

"This is an act of sabotage," Interior Minister Marwan Charbel said,

adding: "We cannot at this stage accuse anyone" of being behind the attack.

"We hope that what is happening in Syria will not spill over into Lebanon," he told journalists.

The security source said the rockets were fired from Aitat in the Mount Lebanon area, some 13 kilometres (8 miles) southeast of where they hit, and the army found two abandoned rocket launchers there.

"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 told the group's Al-Manar television.

French Foreign Minister Laurent Fabius, speaking in Abu Dhabi, strongly condemned the Beirut attack, stressing it was crucial to "avoid the war in Syria becoming a war in Lebanon."

The fighting in Syria has already spilled over into Lebanon's northern port city of Tripoli, where 30 people have been killed in a week of clashes between pro-Assad Alawites and pro-rebel Sunni Muslims.

The intervention of hundreds of Hezbollah fighters has given Assad the upper hand in Qusayr, a strategic central town in Syria across the border with Lebanon, that had been in rebel hands.

Syrian forces launched an assault on Qusayr on May 19 but are still being fiercely resisted, as the town provides an important rebel supply line for arms and volunteers from Lebanon.

Nasrallah's victory pledge followed rare criticism of Hezbollah by Lebanese President Michel Sleiman, who cautioned it against its intervention in Syria.

"Those behind this attack are terrorists and vandals who want neither peace nor stability for Lebanon and the Lebanese," a presidency statement said after Sunday's rocket attack.

The bodies of nine Hezbollah fighters killed at Qusayr were returned to Lebanon on Saturday, their families said.

The Syrian Observatory for Human Rights said fighting in Qusayr on Saturday killed at least 27 rebels and three civilians, including a child.

Qusayr is a key prize for Assad because of its strategic location between Damascus and the mainly Alawite Mediterranean coast.

The main opposition Syrian National Coalition in a statement on Sunday urged Hezbollah fighters in Syria to defect.

It was meeting in Istanbul again to try to overcome deep divisions over Russian and U.S. proposals for a peace conference.

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.

Delegates said efforts to reach an agreed position on the conference were being delayed by pressure from some of the opposition's 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," a Coalition member said on condition of anonymity.

"Their goal is to downsize the Muslim Brotherhood's influence over the group," he added.

The National Coalition is currently dominated by the Syrian National Council, in which the main political bloc is the Muslim Brotherhood.

The Coalition, wrong-footed by Moscow's announcement that regime representatives had agreed to attend next month's planned peace conference, urged Damascus to give concrete evidence of its readiness for a transition of power.

Syrian Foreign Minister Walid Muallem was in Baghdad for talks on Sunday, a day after Iraq launched a massive operation to better secure its western desert amid concerns it is being used by militants heading to fight Assad's regime.

The United States and Russia, which support opposite sides in Syria's conflict, are pushing for the conference. On Monday, US Secretary of State John Kerry will meet his Russian counterpart Sergei Lavrov in Paris to step up efforts to organize it.