BEIRUT - A powerful car bomb struck the southern Beirut stronghold of Lebanon's militant Hezbollah group on Thursday, killing 20 people, wounding 226 and trapping many others inside damaged buildings, witnesses and emergency officials said.

The blast, a month after a car bomb injured more than 50 people in the same district of the Lebanese capital, came amid sectarian tensions over the intervention of Shi'ite Muslim Hezbollah against Sunni rebels in Syria's civil war.

A Hezbollah supporter tries to extinguish the fire from burning cars as people gather at the site of an explosion in Beirut's southern suburbs, August 15, 2013. Twenty people were killed in the explosion which struck the southern Beirut stronghold of Lebanon's militant Hezbollah group on Thursday, a security source said. An army statement said the cause of the explosion was a car bomb. The blast engulfed several vehicles in flames and trapped many people in nearby buildings which were damaged in the explosion. [Photo/Agencies]

A Sunni Islamist group calling itself the Brigades of Aisha claimed responsibility for the attack and promised more operations against Hezbollah. It was not immediately possible to verify the statement, which was made in an internet video.

"I don't know what happened. It's as if we were struck by an earthquake," one young man at the scene told Reuters, bleeding from a stomach wound.

Health Minister Ali Hassan Khalil said hospitals across the capital had taken in 16 bodies and 226 wounded people.

The UN Security Council strongly condemned the attack.

"The members of the Security Council appealed to all Lebanese people to preserve national unity in the face of attempts to undermine the country's stability," the 15-member council said in a statement.

At the heart of the site, where fires raged an hour after the blast, the twisted remains of a large van could be seen.

Many cars were engulfed in flames, the charred bodies of drivers and passengers visible inside. The blast sent a column of black smoke above the densely populated area and the facades of several residential buildings were damaged.

Al Mayadeen television said some people were trapped inside apartments at the scene, close to the Sayyed al-Shuhadaa (Martyears) complex, where Hezbollah leader Sayyed Hassan Nasrallah often addresses his followers.

Residents of southern Beirut say Hezbollah, backed by Iran and Syria, had been on high alert and stepped up security in the area after warnings from Syrian rebels of possible retaliation for the group's support for President Bashar al-Assad.

"I heard a huge explosion. It threw me several metres," said a woman in her 50s who said she had been talking to her brother in his shop.

"I don't know what happened to my brother. I can't find him," she said, bleeding from wounds to hands and face.