Hezbollah strongholds in the Lebanese capital Beirut have been struck by two rockets hours after the Shiite militant group's leader Hassan Nasrallah pledged to extend full support to Syrian President Bashar al-Assad.

At least four people were injured in the attack which targeted the southern suburb of Shiyya, said witnesses. The neighbourhood houses some of the main offices of Hezbollah.

One of the rockets hit a car showroom while the other struck a residential building, suggest reports. One rocket failed to explode, Hezbollah-backed al-Manar TV reports.

The launchpads for the attack have been discovered by Lebanese troops and the army has opened an investigation into the attack.

Interim Interior Minister of Lebanon Marwan Charbel described the attack as an "act of sabotage". He said after inspecting the affected area: "The rockets were fired from a secluded area, southeast of the scene of the incident.

"I believe this was an act of sabotage. I hope what is happening in Syria will not happen in Lebanon."

However, Charbel refused to speculate whether the rebels fighting against Assad are behind the attack.

The rocket attack is believed to be a response to Nasrallah's remarks in which he pledged strong support for Assad until the war is fully over.

The Lebanon-based militant group, widely perceived as the long arm of Iran, has been increasingly involved in the ongoing Syrian civil war during recent weeks. Until recently, the outfit had not formally admitted to sending its fighters into combat in support of Assad.

In his latest address marking the anniversary of Israel's military withdrawal from Lebanon, Nasrallah said: "We are fighting in Syria, you are fighting in Syria and let us continue to fight there. Put Lebanon aside..... I say to all the honourable people, to the mujahedeen, to the heroes: I have always promised you a victory and now I pledge to you a new one."