Violence has intensified around Damascus, the Syrian capital, with reports of shelling by government forces on rebel-controlled towns and fierce street fighting.
President Bashar al-Assad forces shelled the eastern suburb of Douma and the nearby towns of Zabadani and Moadamiyet al-Sham, activists and official sources said on Thursday.
Battles reportedly continued near Damascus International Airport. Activists told Al Jazeera that the main road to the airport has been cut off at several instances when clashes had intensified.
Activists also said fighting was raging near Akraba Military Airport, located near the main road leading to the international airport.
Sources in the capital told Al Jazeera that while opposition fighters tightened their siege of Akraba airport, it remained under the control of regime forces.
'Cleansing of terrorists'
Meanwhile, in the Damascus suburb of Daraya, additional troops were deployed, as the army escalated its assault on rebel-held area.
"Syrian army units continued today to pursue terrorists loyal to Al-Nusra Front, which is part of al-Qaeda, in Daraya," state news agency SANA said.
Citing an unnamed military source, the agency also said "Daraya will be completely cleansed of terrorists soon," using its standard term for rebels.
An activist told the AFP news agency that the army was advancing little by little into Daraya, though the rebel Free Syrian Army was fighting hard to keep the troops out of the town.
"The army has been able to enter into around 30 per cent of Daraya in recent days, and there are serious fears about what would happen to the town if the troops do reclaim it," said the man, who identified himself as Abu Kinan.
In-depth coverage of escalating violence across Syria
Daraya was the scene in August of the single worst massacre in Syria's 21-month conflict, where hundreds were reported killed.
Activists also reported clashes in the eastern suburb of Irbin.
Damascus province is now a key battlefield, as regime forces battle to retake control of an 8km belt around the capital, analysts say.
In the capital itself, security forces were out in force in the southern district of Zahra after a car bomb exploded on Thursday.
State television said "al-Qaeda terrorists exploded a bomb in a car in front of a Red Crescent centre in Damascus, causing one death and major damage."
In the embattled northern city of Aleppo, several districts saw clashes, while shells slammed into zones in the southern province of Daraa.
The Syrian Observatory for Human Rights said the violence on Thursday had claimed 104 lives across the country, according to a toll compiled from a network of activists, lawyers and doctors.
The UK-based watchdog has tallied more than 41,000 deaths, most of them civilians, since the uprising erupted in March 2011.
In Dublin, talks between US Secretary of State Hillary Clinton, her Russian counterpart Sergei Lavrov and UN peace envoy Lakhdar Brahimi failed to reach any "sensational decisions".
Brahimi said that, during the meeting on the sidelines of an international gathering, all three agreed the situation was "very, very, very bad" in Syria.
The three discussed "how we can work out hopefully a process that will get Syria back from the brink," Brahimi said.
The US has been calling on Russia for some time to use its leverage with Assad to try to open the way towards a political transition, although Washington has insisted the long-time leader will have to go.
US officials had hoped Lavrov was signalling a new willingness by staunch Damascus ally Moscow, by accepting Brahimi's invitation, to probe ways to bring more pressure to bear on Assad to step down.
Brahimi said afterwards the three agreed to put together a peace process that will be based on a Geneva accord, adopted under the previous joint UN-Arab League envoy Kofi Annan.
In London, meanwhile, David Lidington, UK's minister of European affairs, said his country will press its European partners next week to amend the arms embargo on Syria to allow them to provide weapons to rebels fighting the regime.
The EU agreed late last month to extend its sanctions against Assad's regime, including the arms embargo, for a further three months, after they were due to expire on November 30.
"We will make fresh arguments in support of amending the arms embargo ahead of the March 2013 deadline in a way that offers sufficient flexibility to increase practical support to the Syrian opposition," Lidington said in a statement.
Britain has formally recognised a newly formed opposition bloc as the sole representative of the Syrian people , but this cannot include weapons at the moment because of the EU embargo.