Rocket attacks struck government-held districts in the central Syrian city of Homs on Thursday, setting off successive explosions in a weapons depot that killed at least 40 people and wounded dozens, an opposition group and residents said.

The blasts sent a massive ball of fire into the sky, causing widespread damage and panic among residents, many of whom are supporters of President Bashar Assad.

One resident said the explosions were so strong they cracked the walls of some buildings. Thick smoke and dust could be seen from a distance as explosions shook the ground.

A video posted online by activists showed a huge ball of fire over Homs neighborhoods.

The explosions in Homs reflected the see-saw nature of the conflict. It showed that despite significant advances by Assad's military, rebels could still strike back.

An official at the governor's office in Homs said about 10 rockets slammed into the neighborhood of Zahra and the nearby sports stadium, sparking a large fire and causing several casualties. He said the explosions caused massive destruction and wounded at least 130 people. He didn't offer a number for those killed.

The official spoke on condition of anonymity in line with government regulations. He did not give a casualty figure.

The Britain-based Syrian Observatory for Human Rights, which closely monitors the fighting in Syria, said 40 people were killed and 120 were wounded — some critically — when rockets struck an arms depot, igniting the fire.

A resident of Homs corroborated that account, saying he heard blasts for more than an hour after the first explosion. He said they could be heard from the overwhelmingly pro-regime districts of Wadi Dahab and al-Walid, where the regime is known to keep arms depots.

"Rockets were falling on the area ... when the arms depot began to explode but we don't know if the rockets triggered the blasts," he said, speaking on condition of anonymity for fear of retaliation.

He said the explosions were so strong that "they shook parts of the city." They also shattered all windows in the area and cracked walls, he added.

He said he spoke with Syrian Arab Red Crescent paramedics who told him that at least 22 bodies were taken to hospitals.

The explosions in Homs coincided with a rare trip by President Bashar Assad to a former rebel bastion near the capital, Damascus, to mark Army Day.

Assad's visit to Daraya is his first known public trip outside the capital, his seat of power, in more than a year. He visited the battered Baba Amr district in the central city of Homs after troops seized it from rebels in March 2012.

It also is the latest sign of confidence from Assad, whose troops have been on the offensive and scored significant gains against rebels in recent months. Assad pledged victory over troops fighting to topple him.

More than 100,000 people have been killed since the uprising against the Assad family's four-decade rule began in March 2011. The revolt later escalated into a civil war, which has uprooted millions of people from their homes.