Syria's interior ministry has vowed to "strike back with an iron fist" at what it say is a recent "escalation" of anti-government terror attacks.

It comes after a bomb in the capital, Damascus, killed at least 25 civilians and members of the security forces on Jan 6.

Opposition groups have accused the government of planting the bomb to discredit their opponents and influence Arab League monitors.

The monitors are assessing the progress of a regional peace plan. But activists say the government crackdown has continued, with scores of people killed since the monitors arrived.

The UN says more than 5,000 civilians have been killed since protests against President Bashar al-Assad began 10 months ago.

The Damascus blast happened at a busy junction in the Midan district of Damascus. State TV showed the shattered blood-stained windows of what appeared to be a bus carrying policemen.

Authorities say most of those killed were civilians, but some security personnel were among the casualties, BBC News reported.