The Syrian Observatory for Human Rights in London said explosives were placed in a building on a government military base in the Damascus suburb, The New York Times reported. The building was flattened by the blast, the anti-Assad regime group said.
The group said the bomb appeared to have been placed inside the building or in a tunnel.
Government officials and state media had not confirmed the attack, the newspaper said.
The Times said the attack may have been an attempt by Syrian rebels seeking to topple President Bashar Assad to counter his military's recent advances.