Fierce fighting has been reported between Syrian troops and rebel forces in the suburbs of the capital Damascus.
Witnesses say it is some of the most intense violence in the area since the uprising against President Bashar al-Assad began more than a year ago.
Reports from activists, which cannot be verified, say rebels clashed with Syria's elite Republican Guard.
The violence came as Turkey issued a stark warning to its neighbour Syria over the downing of a jet last week.
Prime Minister Recep Tayyip Erdogan told parliament that if Syrian troops approached Turkey's borders, they would be seen as a military threat.
In other developments on Tuesday:
- President Assad told his new cabinet Syria is in a "state of war" and that all government efforts should be directed at winning it, the state news agency reported
- The head of UN peacekeeping operations, Herve Ladsous, said the monitoring mission in Syria would remain suspended because of mounting violence
- Russia said its foreign minister Sergei Lavrov would attend an international conference on Syria which special envoy Kofi Annan hopes to hold in Geneva on 30 June
The UK-based Syrian Observatory for Human Rights said that clashes happened overnight and into Tuesday near Republican Guard positions in Qadsaya and al-Hama, around 8km (5 miles) from the centre of Damascus.
Correspondents say it is rare for fighting to take place near Republican Guard bases and suggests a growing confidence among the rebels.
The elite Republican Guard, commanded by President Bashar al-Assad's younger brother Maher, is tasked with protecting the capital.
State TV confirmed the fighting but said dozens of "terrorists" had been killed and many others taken prisoner, including foreign fighters.
It said large numbers of armed rebels had moved into al-Hama and tried to take control of a main road to the west in order to bring in more arms and fighters.
The Observatory said that 10 people had been killed by shelling in Qadsaya and some 58 people had died in violence across Syria on Tuesday - 24 soldiers, 30 civilians and four rebels. The figures cannot be independently verified.
Observatory head Rami Abdel Rahman told AFP: "This is the first time that the regime has used artillery in fighting so close to the capital.
"This development is important because it's the heaviest fighting in the area and close to the heart of the capital."
Heavy shelling was also reported in Homs, where the International Committee of the Red Cross (ICRC) last week tried unsuccessfully to arrange the evacuation of civilians.
The ICRC said on Tuesday it was returning to the city for a fresh attempt.
Earlier, Mr Erdogan spoke of Turkey's "rage" at Syria's decision to shoot down the F-4 Phantom last Friday and described Syria as a "clear and present threat".
"A short-term border violation can never be a pretext for an attack," he said.
He said the Turkish jet had been on a training flight, testing Turkey's radars in the eastern Mediterranean.
Mr Erdogan said Turkey was adopting a "common sense" attitude, but that "shouldn't be perceived as a weakness".
"Every military element approaching Turkey from the Syrian border and representing a security risk and danger will be assessed as a military threat and will be treated as a military target," he said.
Syria insists the F-4 Phantom was shot down inside Syrian airspace.
Nato, of which Turkey is a member, convened an emergency meeting of its ambassadors on Monday and afterwards expressed "strong solidarity" with Ankara.
Relations between Syria and Turkey were already highly strained before the F-4 was shot down.
The jet crashed into the eastern Mediterranean and its two pilots are missing.
The Russian foreign ministry said on its website on Tuesday that the downing of the plane should not be seen "as a provocation or a premeditated action".
Russia is a close ally of Syria and, together with China, has blocked UN Security Council resolutions condemning Damascus for the continuing violence.