Dozens of Syrians were kidnapped and Syrian-owned shops vandalised in Beirut on Wednesday, Lebanon's state news agency said, after unconfirmed reports that several Lebanese taken hostage in Syria had been killed.

Gunmen have taken to the streets in the Shiite southern suburb of Tiro, the National News Agency said.

"Some of the attackers vandalised shops, destroyed cars for sale, and kidnapped dozens of Syrians," it said. "The situation has got out of control."

The violence erupted after unconfirmed reports were broadcast claiming several Lebanese kidnapped in Syria in May were killed in an air strike in the northern town of Aazaz on Wednesday.

According to the Syrian Observatory for Human Rights, seven of the 11 Lebanese Shiite pilgrims kidnapped in Syria were injured, but none was killed.

Violence in neighbouring Syria has often spilled over into Lebanon in the form of cross-border shootings, shelling by the Syrian army, tit-for-tat kidnappings and sectarian clashes between groups which are divided over the revolt.

"The relatives and neighbours of several Lebanese kidnapped in Syria took to the streets, and started to harass Syrians and vandalise their property in the area," the NNA said.

Syria's main opposition coalition condemned the kidnappings.

"The Syrian National Council expresses its deep dismay at the kidnapping of a large number of Syrian citizens, who had fled to Lebanon to take refuge from bloody oppression in their country," it said in a statement.

Earlier Wednesday, a large Lebanese Shiite clan said it kidnapped at least 20 Syrians to try to secure the release of a family member abducted near Damascus this week.

"We do not take part in any harassment of innocent Syrians, but any supporter of the Free Syrian Army is a legitimate target for us," Hatem al-Muqdad told Lebanese television station LBC.

Syrian rebel groups have accused Hezbollah—which is closely allied with the regime of President Bashar al-Assad—of playing a role in the violence, though the group has consistently denied this.

Syria occupied Lebanon militarily and politically for nearly three decades until 2005, when its troops were forced to pull out under international pressure after the assassination of former Lebanese premier Rafiq Hariri that year.

Seven years after Syria withdrew from Lebanon, the country's political forces remain sharply divided over events in their neighbour.