Government troops have stormed a Damascus district with tanks for the first time, five days on from the outbreak of
fierce clashes in the capital, a UK-based rights group has said.
"The army stormed the Qaboon district with a large number of tanks," said Rami Abdel Rahman, of the Syrian Observatory for Human Rights (SOHR), on Thursday.
"This is the first time that tanks enter a Damascus district," Rahman said.
The army's move stoked fears of an imminent massacre in the western quarter of the capital, scene of clashes over the past five days, the SOHR said.
The intensity of ongoing fighting in Damascus was underscored on Wednesday by a devastating bomb attack at the heart of Syria's senior command that killed at least three of President Bashar al-Assad's top brass
Earlier on Thursday, a security source told the AFP news agency on condition of anonymity that the army would show no restraint in its operations.
"These extremely violent clashes should continue in the next 48 hours to cleanse Damascus of terrorists by the time Ramadan begins" on Friday, the source said, referring to the Muslim holy month.
The developments came a day after three top regime officials were killed in an unprecedented attack in the National Security headquarters in Damascus, which was claimed by the rebel Free Syrian Army (FSA).
"After the attack, [the army] has decided to use all the weapons in its possession to finish the terrorists off," the security source said.
Violence killed at least 107 people across Syria on Thursday, the SOHR said, and forced hundreds of Damascus residents to flee their homes for safer neighbourhoods.
Earlier on Thursday, opposition fighters attacked the main police headquarters in Damascus on Thursday, a witness said.
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"Gunfire has been intense for the past hour. It is now dying down but the streets around the police command remain empty," a resident of Qanawat, an old central district where the Damascus Province Police Headquarters is located, told Reuters by phone.
Intense fighting between the opposition and government forces also raged in a half-dozen areas of the Syrian capital Damascus.
Columns of black smoke rose over Damascus on Thursday as troops shelled Qaboon and Barzeh, while fighting raged in al-Midan and Zahira and loud explosions were heard in Mashrou-Dumar, said the Syrian Local Co-ordination Committees.
Violence also erupted in Ikhlas neighbourhood near the government headquarters after rebels attacked forces loyal to Assad, who have deployed armoured vehicles, attack helicopters and increased roadblocks across the city.
Rebels fired rocket-propelled grenades at a police station in the Jdeidet Artouz area, killing at least five officers, the SOHR said.
Al Jazeera's Zeina Khodr, reporting from Beirut, said many people believed the latest developments had brought the Syrian conflict to a turning point.
"The Damascus fighting is now in its fifth day, getting close to power base of the Syrian president," our correspondent said. "The prestige of the regime has been shattered. Losing control of Damascus [means] the regime is slowly losing its grip over the country."
More than 200 people, mostly civilians, were killed on Wednesday, including 38 in Damascus, where rebels are pressing an all-out offensive, according to the SOHR.
Wednesday's bombing killed Defence Minister General Daoud Rajha, Assad's brother-in-law Assef Shawkat and General Hassan Turkmani, head of the regime's crisis cell on the uprising, state media said.
Among those wounded were Interior Minister Mohammed al-Shaar and General Hisham Ikhtiyar, head of national security.
Conflicting accounts have emerged of who carried out the attack on Wednesday and how it was perpetrated.
Patty Culhane reports on the UN's next steps
Syrian state media did not air any images of the blast, as in previous explosions that hit Damascus in the last two months.
Within hours the government named Major General Fahad Jassim Feraj as defence minister, the state news agency SANA reported.
The attack was claimed by the FSA, although another group, the Brigade of Islam, also said it was responsible.
The rebels said the attack, part of Operation Damascus Volcano launched on Monday, "is the first in a series ... aimed at bringing down Assad and the pillars and symbols of the regime, whether civilian or military".
State media initially said it was a "suicide bombing" before apparently retracting and calling it a "terrorist attack".