Story highlights 18 killed, 291 wounded, state news says

The army says a car bomb caused the blast

An unknown group claims responsibility in a YouTube video

The area is a Hezbollah stronghold

A car bomb Thursday rocked a southern suburb of Lebanon's capital -- an area known as a stronghold of the Lebanese Shiite militant group Hezbollah, the Lebanese Army said.

Eighteen people were killed and 291 were wounded, the Lebanese national news agency NNA reported.

Video shown on Hezbollah's Al-Manar TV showed fierce flames engulfing several cars in the Beirut suburb. Thick, black smoke rose from the vehicles; a nearby building also appeared to be on fire. Ambulances were rushing to the site of the blast, NNA reported.

"Army units arrived immediately at the scene, enforcing a security zone within the area of explosion while military experts initiated investigations under the concerned judiciary's surveillance," the army said.

After the blast, a YouTube video surfaced in which three masked men holding weapons claimed they are part of a group responsible for the attack. They stood before a large Arabic sign containing the Islamic creed, "There is no God but Allah, and Mohammed is the messenger of Allah."

JUST WATCHED July: Massive car bomb in Beirut Replay More Videos ... MUST WATCH July: Massive car bomb in Beirut 02:55

The group, which calls itself "The brigade of Aisha, Mother of Believers," accused Hezbollah of being aggressors. Aisha is the last wife of the Prophet Muhammad and is especially revered by Sunni Muslims.

In its YouTube post, the group called Hezbollah leader Hassan Nasrallah a "pig" and said he works on behalf of Iran and Israel.

"A message: To our brothers and sisters in Lebanon: Stay away from anything to do with Iran -- whether in Beirut or outside Beirut," it said.

Hezbollah is a Shiite movement in Lebanon. Along with Shiite-dominated Iran, the group is helping Syria's government fight rebels, whose ranks are dominated by Sunnis.

CNN could not confirm the authenticity of the video or the men's claims. The U.S. Embassy in Beirut tweeted condemnations of the attack.

Civil Defense units were trying to rescue some citizens stuck inside their homes due to the fire resulting from the blast, NNA reported.