Syrian rebels claim that they have shot down a government warplane in the eastern province of Deir Ezzor.
"The rebels shot down the plane and captured its pilot," the Local Co-ordination Committees (LCC) activist network said on Monday.
The LCC posted videos on YouTube that they allege is of the jet being downed, and then another of the plane's pilot giving a statement. Al Jazeera cannot verify the content of the videos.
The state news channel Syria TV said the plane crashed due to technical problems during a "regular training mission" and a search party was under way.
State news agency SANA said the pilot had ejected from the plane before it crashed.
Speaking to Al Jazeera, Ammar Waqqaf, from the Syrian Social Club, a London-based group supporting President Bashar al-Assad, said the Syrian government remained "fairly stable".
"We've seen today a loss of an air plane, whether for technical reasons or whether it was really shot down, we don't know," said Waqqaf.
"The army is pretty much intact. They're carrying out operations against the insurgency."
He dismissed a wave of recent defections as "some defections hat have been orchestrated, most probably, by some sort of covert intelligence operations".
On the outskirts of Damascus, the capital, shells struck rebel strongholds from before dawn on Monday.
Activists said more than 45 people, including 36 civilians, had been killed in the past 48 hours in the area.
Security forces reportedly arrested at least 21 people as they raided houses and shops in the centre of Damascus on Monday.
The Syrian Revolution General Council, a network of activists on the ground, said that armed forces had broken down the doors of shops that had closed in a show of defiance against the regime.
Activists said the raid was the biggest operation of its kind in the city since the launch of the uprising against Assad's regime in March last year.
Rami Abdel Rahman, of the UK-based Syrian Observatory for Human Rights, said: "This is not the first time there have been raids in central Damascus, but this is the first time something like this has happened on such a large scale."
Security forces also swept into a graveyard "under the pretext of searching for weapons", he said.
The group also reported that several houses had been burned in Basateen al Mezzeh, in the capital, and that there were additional clashes in the outskirts of Damascus, where armed rebels were able to destroy some heavy military vehicles.
Fighting also raged in the city of Aleppo, with rebels seeking to establish a strategic foothold in the north.
Clashes were reported in the southwestern district of Salaheddin, from which rebels fled last week.
Syrian troops were advancing on the southwestern rebel stronghold of Sukari, security sources said.
Activists on Sunday said "communications of all forms" were cut in most of Aleppo city and its suburbs.
On Monday, UN humanitarian chief Valerie Amos announced that she is to visit Syria and Lebanon amid growing concern over the "deteriorating humanitarian situation".
In-depth coverage of escalating violence across Syria
The visit, planned from August 14 to 16, "aims to draw attention to the deteriorating humanitarian situation in Syria and the impact of the conflict on people either remaining in Syria and who have fled to other countries, including Lebanon", the UN office for the coordination of humanitarian affairs said in a statement.
While in Syria, Amos is expected to discuss ways of "urgently scaling-up relief efforts and reducing the suffering of civilians caught up in the fighting with the Syrian authorities, the Syrian Arab Red Crescent and other humanitarian partners".
The Organisation of Islamic Co-operation (OIC) summit will be held in Mecca on Tuesday.
An unnamed diplomat there told Reuters news agency that Syria will "most likely" be suspended from the OIC, a move Iran opposes.
"By suspending the membership, this does not mean you are moving towards resolving an issue. By this, you are erasing the issue," Iranian Foreign Minister Ali Akbar Salehi told reporters in Jeddah, seat of the 57-strong OIC of which allies Iran and Syria are members.
"Every country, especially OIC countries must join hands to resolve this issue in such a way that will help the peace security and stability in the region," he added.
Iranian President Mahmoud Ahmadinejad has arrived in Saudi Arabia and will attend the summit.