Forty people have been killed and at least 160 more wounded in explosions at a weapons cache in the central Syrian city of Homs, according to an activist group opposed to President Bashar al-Assad.

The blast occurred in the southeastern district of Wadi al-Dhahab on Thursday, which the army has taken over, the Syrian Observatory for Human Rights (SOHR) said.

The group has a network of sources in the opposition and state security forces.

The SOHR said the casualties were soldiers and civilians and that some of the wounded were in a critical condition.

Activists told Al Jazeera that the explosions were a result of rebel shelling of the Wadi al-Dahab area.

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One activist said the blasts were "massive" and were heard from miles away. He could not confirm the target was the city's arms depot.

One resident from the area said the sound of multiple explosions could be heard for over an hour as munitions were detonated.

A rebel in Homs said that the blast was caused by a surface-to-surface Grad missile, according to the Reuters news agency.

He would not say which rebel group fired the missile.

The explosion came after Assad expressed confidence that his regime's forces would be victorious in the 28-month-old conflict with rebels.

He made the comments while touring Daraya, a former rebel-held area near the capital Damascus.

Assad visited the town after the military said it had taken over the strategic district of al-Khalideya in Homs from opposition forces.

Source: Al Jazeera and agencies