It comes after bomb in the capital, Damascus, killed at least 26 civilians and members of the security forces.
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.
Interior Minister Ibrahim al-Shaar blamed the attack on a suicide bomber, who he said had «detonated himself with the aim of killing the largest number of people».
«We will strike back with an iron fist at anyone tempted to tamper with the security of the country or its citizens,» he said.
The country’s main opposition coalition, the Syrian National Council (SNC) said the attacks had been carried out by Mr Assad’s government to discredit its critics.
«It is a continuation of the regime’s dirty game as it tries to divert attention from massive protests,» said spokesman Omar Idlibi.
«We call upon for an independent international committee to investigate these crimes that we believe that the regime planned and carried out.»
Maj Maher al-Naimi, a spokesman for the armed anti-government movement the Free Syrian Army (FSA), said the blast was «planned and systematic state terrorism by the security forces of President Bashar al-Assad».
FSA leader Col Riad al-Asaad reportedly denied involvement in the attack.
A British journalist who visited the scene told the BBC reporters were not shown any bodies of those killed.
«We left with the sense of this is a horrific spectacle but that maybe some of the details weren’t quite as one might have expected,» said Ian Black of the Guardian newspaper.
The US condemned the attack, saying violence was not «the right answer to the problems in Syria».
Two weeks ago 44 people died in similar blasts also blamed on terrorists but which opposition groups accused the government of staging.
Arab League concern
Meanwhile, activists reported further violence on Friday. Anti-government protests have regularly followed traditional Friday prayers.
The LCC said nine protesters had been killed in Hama, 14 in the suburbs of Damascus, eight in Homs, three in Idlib and one in Deraa. The numbers cannot be verified.
The Arab League observers have been in Syria since late December to monitor compliance with a peace plan under which the government promised to withdraw the military from the streets and cease its use of force against civilians.
But critics say Mr Assad is using their presence as a political cover and that attacks continue.
Arab League Deputy Secretary-General Ahmed bin Helli said he was «concerned» about the Damascus blasts.
«That is why we are calling on the Syrian government to be totally cooperative with the mission and to work by all means to stop the bloodshed and allow room for the political process to begin,» he told the Associated Press.
He said observers at the scene would «undoubtedly have an opinion» about what had happened.
Opposition activists have urged Syrians to take to the streets in mass protests ahead of an Arab League meeting in Cairo on Sunday which will debate the initial findings of the observer mission.