Two rockets have hit a district in the southern part of Lebanon's capital, wounding at least five people.

One rocket struck a car dealership in the Mar Mikhael district on the edge of Beirut on Sunday, injuring Syrian workers, Lebanese security officials said.

Another rocket hit the second floor of an apartment building in the Shiyah district, about 2km away, without causing any injuries.

The area is a stronghold of the Lebanese armed group Hezbollah, which is backing government troops in neighbouring Syria in the battle against rebels seeking to topple President Bashar al-Assad.

Sunday's attack was the first time Beirut's mainly Shia southern suburbs have been targeted during the two-year-old Syrian conflict, and came just a day after a televised speech in which Hassan Nasrallah, Hezbollah's leader, affirmed his commitment to fighting in the war.

Marwan Sharbel, Lebanon's interior minister, visited the area where the rockets fell, saying they were fired from the southeastern side of the area but that it was too early to know who fired them.

It was an act of "sabotage", he said.

The army said it found two abandoned rocket launchers in woods near Aitat in the Mount Lebanon area, about 13km southeast of where they hit.

Lebanon's state agency said security forces were looking for a third rocket that fell but did not explode.

Call for investigation

Syrian rebels have threatened to retaliate against Hezbollah for sending fighters to assist government forces.

Al Jazeera's Rula Amin, reporting from Beirut, said security forces surrounded the area where the rockets fell.

"The attack does not come as a surprise and security forces had already been heavily deployed in the area in anticipation for any violence," she said.

Hasan Fadlallah, a Hezbollah parliamentarian, said the group would "confront any attack against it from whatever side or place it comes from".

"We call for a quick and serious investigation to uncover the identity of the criminals who committed such an attack and those who supported them in order to bring them to justice," he said.

Nasrallah said, in his speech on Saturday, that his fighters were battling in Syria against religious radicals who posed a danger to Lebanon, and pledged that his group would not allow Syrian rebels to control areas along the Lebanese border.

His speech offered the clearest public confirmation yet that Hezbollah, which is backed by Iran, was directly involved in Syria's war.

'Dark period'

Hezbollah initially tried to play down its involvement, but could no longer do so after dozens of its fighters were killed in the Syrian border town of Qusayr and buried in large funerals in Lebanon.

Nasrallah's speech came as battles raged in Qusayr under fire for a seventh straight day as government forces backed by the Hezbollah fighters battled to drive out Syrian rebels.

At least 40 people were reported killed on Saturday in what residents told Al Jazeera was the worst fighting they had seen since the start of the civil war.

Nasrallah promised "victory" in Syria, warning that the fall of the Damascus regime would plunge the Middle East into a "dark period".

He pledged that Hezbollah would turn the tide of the conflict in Assad's favour, and stay as long as necessary to do so.

"We will continue this road until the end, we will take the responsibility and we will make all the sacrifices," he said.

"We will be victorious."

The fighting in Syria has already spilled over into Lebanon's second city, the northern port of Tripoli, where 31 people have been killed and 212 wounded in a week of clashes between pro-Assad Alawites and pro-rebel Sunni Muslims.