Forty people were killed and at least 120 people were wounded in an explosion at a weapons cache in the central Syrian city of Homs.
A group opposed to President Bashar al-Assad, the Syrian Observatory for Human Rights said the explosion occurred in the south-eastern district of Wadi al-Dhahab, which the army has taken.
The group has a network of sources in the opposition and state security forces.
The observatory said the casualties were soldiers and civilians and that some of the wounded were in a critical condition.
Meanwhile UN inspectors are preparing to travel to Syria within days to investigate claims of chemical weapons use in the country's civil war after the Syrian government granted access to three sites.
"The team will depart for Syria as soon as practical and is preparing to depart within days," UN spokesman Martin Nesirky told reporters.
The United Nations has received 13 reports of chemical weapons attacks.
The Syrian government and the opposition both deny using such weapons.
The UN inquiry will only try to establish if chemical weapons were used, not who used them.
