Syrian rebels claimed they downed a fighter jet in what would be a major coup for the opposition, but the regime seized the upper hand in Aleppo as it advanced into a new rebel-held district.
President Bashar al-Assad's regime meanwhile faced mounting pressure as foreign ministers called for Syria to be suspended from the 57-nation Islamic Cooperation Organization over the brutal crackdown on the 17-month rebellion.
But the Western-backed rebels could soon face similar criticism, after shocking videos emerged showing alleged rebel atrocities in Aleppo, the scene of several days of fierce urban combat and thundering air strikes.
State media said a military plane on a training mission crashed in the east of the country after suffering a malfunction and that the pilot had ejected.
But the Free Syrian Army, which has been requesting anti-aircraft weapons so that it can respond to escalating attacks from the sky, claimed it shot down the Russian-made jet in the eastern province of Deir Ezzor.
"With God's help, a MiG 23 plane was downed in Muhasen on Monday morning, by the hands of the Ahfad Mohammed (Grandchildren of Mohammed) Brigade," said Captain Abul Laith, whose group is part of the FSA.
Another group calling itself the "Revolutionary Youth of the Land of the Euphrates" distributed a video showing a man identified as pilot Mufid Mohammed Suleiman surrounded by three armed men.
"My mission was to bomb the town of Muhasen," said the purported pilot.
FSA spokesman Kassem Saadeddine identified the pilot as Colonel Mufid Mohammed Sleiman and said he was a member of the Alawite minority community of President Bashar al-Assad and his inner circle.
"He is a member of a squadron I used to fly with... before I defected," Saadeddine told AFP via Skype. "I served with him for 15 years, and he is a staunch enemy of the revolution."
If confirmed, the attack would be the first time the rebels have succeeded in downing a Syrian plane since Assad's regime launched an increasingly brutal crackdown on protests 17 months ago.
International concern is mounting over how to end a conflict that has triggered a major humanitarian crisis and sent around 140,000 Syrians fleeing to neighbouring countries, with scores of people being killed every day.
At least 103 lost their lives on Monday, 58 of them civilians, the Syrian Observatory for Human Rights said.
The opposition has accused the regime of increasingly resorting to firing from fighter planes, particularly on the second city of Aleppo, which has witnessed some of the fiercest fighting in recent weeks.
Opposition umbrella group the Syrian National Council issued a new appeal for the establishment of no-fly zones similar to those set up over Libya during last year's conflict.
SNC head Abdel Basset Sayda told AFP the rebels wanted "two no-fly zones, one in the north near the Turkish border, and another in the south near the border with Jordan."
Signup to our newsletter and follow us on Facebook and Twitter!
Washington said that Assad's government was employing more air power in its war with the rebels.
"We've seen a very troubling and despicable uptick in attacks from the air, perpetrated by the Syrian regime," Pentagon spokesman George Little told reporters.
Asked if the Pentagon was moving towards enforcing a possible no-fly zone, he said: "We plan for contingencies."
White House spokesman Jay Carney said the administration has "ruled out no option" in trying to bring about a political transition in Syria, but did not explicitly refer to a no-fly zone.
Foreign ministers of the Islamic Cooperation Organisation meanwhile called on heads of state meeting in Mecca for an emergency summit on Tuesday to suspend Syria from the 57-nation bloc.
Sources close to the preparatory meeting of the ministers, held in the Saudi Red Sea City of Jeddah, said that only Algeria and Iran -- Syria's closest ally -- were against the recommendation.
"We certainly do not agree agree with the suspension of any OIC member," Iranian Foreign Minister Ali Akbar Salehi said after the meeting.
"We have to look for other ways, means and mechanisms for resolving conflicts and crises," he said.
Grisly footage meanwhile emerged appearing to show rebels in the Aleppo area callously throwing bodies off a post office building, while another video showed a man, blindfolded and bound, as his throat was savagely cut.
In another video, a blindfolded man with his hands tied behind his back is forced down onto a pavement in Aleppo, calling out: "I would rather die by a bullet."
As the group chants "Allahu Akbar" (God is Greatest) an assailant carves away at his throat with a small blade as his blood spurts onto the pavement.
Both sides in the increasingly vicious conflict have been accused of human rights violations as reports of cold-blooded killings mount, although the authenticity of the latest videos could not be verified.
The army gained some ground as it advanced into another rebel-held area of Aleppo, the northern city seen as pivotal to the outcome of the conflict.
"With tanks, Syria's regime forces have stormed the west of the district of Saif al-Dawla," said the Observatory.
"They are now clashing with the rebels, and parts of Salaheddin are being shelled."
Rebels in July took over several districts but regime forces last week reclaimed most of the Salaheddin district that neighbours Saif al-Dawla.
More than 21,000 people have been killed since March last year, with fighting escalating after the failure of outgoing international envoy Kofi Annan's peace plan and the regime hit by an increasing number of defections by high-ranking officials.