Rebels have targeted Syria's army headquarters with two large bomb blasts in the centre of Damascus.
Military officials say one of the bombs had caused damage to the main building of the General Staff.
Clashes in the general staff compound in the capital's Umayyad Square district left dead on both sides, the Syrian Observatory for Human Rights says.
Iran's Press TV said one of its female reporters had been killed by sniper fire in the capital as the fighting broke out.
"Press TV correspondent Maya Nasser has been killed by sniper fire in the Syrian capital, Damascus," Press TV said in a bulletin on its news website.
The anti-regime Free Syrian Army says it carried out the attack and claimed dozens had been killed. The army says no-one had died.
A BBC reporter in the city says gunfire could still be heard, amid reports that a fierce gun battle had broken out.
Syrian state TV quoted a military source saying no senior personnel were harmed by the explosions, but some of the building's guards were wounded, the BBC reports.
It says "terrorist attackers" had opened fire inside the perimeter of the compound and in nearby streets, and security forces had confronted them.
The blasts happened just before 7am local time (in an area dominated by government buildings.
Diplomats living close to the area say the blasts were the biggest they had heard for months.
The BBC's Jim Muir says President Bashar al-Assad would have been able to hear the blasts from his nearby palace.