At least 19 members of Syria's security forces have been killed in two suicide car bombings targeting a military intelligence headquarters in the south-central city of Palmyra, a watchdog has said.
The Syrian Observatory for Human Rights said that a number of other troops were also wounded, some in critical condition, after Wednesday's simultaneous attacks against the intelligence branch and a nearby security building.
It said rebel fighters attacked the buildings soon after the double blasts, while the army deployed security forces in the city renowned for its Roman ruins and which has been designated as a world heritage site by UNESCO.
A shootout around the intelligence buildings erupted after the bombings leaving eight civilians wounded as clashes broke out elsewhere in the central Homs province city.
Heavy fighting has also erupted in Damascus as rebels launched an offensive against President Bashar al-Assad's forces, breaking a lull in the conflict, opposition activists said.
Authorities in Damascus closed the main Abbasid Square and the Fares al-Khoury thoroughfare as fighters attacked roadblocks and fortifications with rocket-propelled grenades and mortars.
"The areas of Jobar, Zamalka, al-Zablatani and parts of Qaboun and the ring road have become a battleground," activist Fida Mohammad said from the district of Qaboun.
"The first car bomb struck at around six in the morning. The second one, which caused the larger explosion, broke through into the compound 10 minutes later"
- Abu al-Hassan, activist
Another activist said an army tank stationed at the main al-Kabbas roadblock on the ring road had been destroyed.
Residents reported explosions across the east and north of the capital.
In Jobar, a working-class Sunni Muslim area adjacent to Abbasid Square, mosque speakers chanted "God is Greatest" in support of opposition fighters who attacked roadblocks in the neighbourhood, activists told the Reuters news agency.
They said tanks stationed on the edge of the central district of Midan, just outside the walls of Old Damascus, shelled southern districts of the city.
Syrian state television said: "Our noble army is continuing its operations against the fighters in Irbeen, Zamalka and Harasta and Sbeineg, destroying the criminal lairs".
Assad's symbols of power came under attack in Palmyra, 220km northeast of Damascus, on the main road to the
oil-producing east of the country.
Roadblocks 'under attack'
A bomb destroyed part of the back wall of the military intelligence compound near the Roman-era ruins in the city and
then a suicide car bomber drove through, detonating the vehicle and destroying parts of the facility, activists in Palmyra said.
Video footage, which could not be immediately verified, showed a large cloud of thick smoke rising in the city.
In-depth coverage of escalating violence across Syria
"The first car bomb struck at around six in the morning. The second one, which caused the larger explosion, broke through into the compound 10 minutes later," activist Abu al-Hassan said from the city.
He said tanks stationed in the compound fired shells in response into an adjacent residential neighbourhood, killing
Roadblocks across the city also came under attack.
The state news agency said two "suicide terrorists" blew up cars packed with explosives near a garage in a residential
district, killing and wounding several people. Among those killed was a woman, it said.
Street demonstrations against Assad's rule erupted in Palmyra at the beginning of the revolt almost two years ago. But the army has since tightened control of the city, which is situated near a major oil pipeline junction.
After a failed uprising in the 1980s led by the Muslim Brotherhood against the rule of Assad's father, the late
President Hafez al-Assad, thousands of rebels were executed in a military jail in Palmyra.
The United Nations say more than 60,000 people have been killed since conflict started in March 2011.
At least 700,000 Syrians have fled their homes, seeking shelter in neighbouring countries like Jordan, Lebanon and Turkey, and more than one million people have been displaced within Syria during 22 months of fighting, according to aid agencies.