The Britain-based Observatory for Human Rights says the blast Wednesday morning in the city of Palmyra targeted a compound that houses a military intelligence branch and a state security agency.
The group says several regime troops were killed in bombing, which was followed by heavy clashes between rebels and government soldiers guarding the compound.
Syrian authorities have not confirmed the attack. There was no immediate claim of responsibility.
Car bombs targeting state institutions have been a hallmark of Islamic militants fighting alongside Syrian rebels aiming to topple President Bashar Assad.