In what could be a major coup for the Syrian opposition, rebels claim to have shot down a regime fighter plane and captured the pilot.
The claims come as the regime appears to have seized the upper hand in Aleppo, while shocking videos have emerged showing alleged rebel atrocities in the Aleppo area.
The Free Syrian Army (FSA), which has been calling for the international community to arm it with anti-aircraft weapons as it battles escalating regime attacks from the sky, claimed it shot down the Russian-made MiG 23 in the eastern province of Deir Ezzor.
Experts say the jet is decked out in the distinctive markings of the Syrian Air Force.
Video footage shows a bright, yellowy orange point of flame almost immediately blossom into a huge ball of light.
Gunners on the ground can be heard celebrating.
The rebels say one member of the two-man crew died and the other has been captured.
A 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,” the purported pilot said.
FSA spokesman Kassem Saadeddine said he knew the pilot and identified him as Colonel Mufid Mohammed Sleiman, 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 said.
“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 Mr Assad’s regime launched an increasingly brutal crackdown on protests 17 months ago.
Syrian state TV has acknowledged the loss of an aircraft, but blames mechanical failure and says rescue teams are searching for the crew.
The incident occurred after umbrella opposition 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.
Council head Abdel Basset Sayda says the rebels want “two no-fly zones: one in the north near the Turkish border, and another in the south near the border with Jordan”, in addition to “safe places for refugees and humanitarian corridors”.
US secretary of state Hillary Clinton said on Sunday she had discussed the possibility of no fly-zones in talks with Turkish foreign minister Ahmet Davutoglu but that the issue needed more consideration.
“You cannot make reasoned decisions without doing intense analysis and operational planning,” she said.
Meanwhile, grisly footage of apparent atrocities in the Aleppo area emerged, appearing to show rebels callously throwing bodies off a post office building, while another video showed a man, blindfolded and bound, as his throat was savagely cut.
A crowd gathered around several bodies crumpled on the ground outside a building said to be in Al-Bab, near Aleppo, before another three victims were hurled, one by one, from the rooftop.
In another video, a blindfolded man with his hands tied behind his back is forced down on to a pavement in Aleppo, calling out: “I would rather die by a bullet.”
As the group chanted “Allahu Akbar” an assailant forced what appeared to be a small knife repeatedly across his throat.
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.
Rami Abdel Rahman, the director of the Syrian Observatory for Human Rights, says he strongly condemns such atrocities, whoever was behind them.
“If these videos are confirmed, such atrocities harm the revolution. They only benefit the regime and the enemies of the revolution,” he said.
Online activists also condemned the killings, which highlight the escalating brutality of a conflict that started out as a peaceful uprising but which has deteriorated into a brutal civil war. The Observatory says at least 21,000 people have been killed countrywide since March last year.
“What is the difference between them and those who kill your children, women and men? This is not a justified reaction at all!” said Akko.
Brothers Mohammed and Ahmed Malas, both dissident artists who live in exile, posted on their Facebook page: “You are killing in the name of God, it seems, but you are killing just like Assad kills. You do not know God.”
In recent weeks, activists have frequently expressed concern about human rights abuses by the rebels, as well as repeated atrocities by the regime since it launched its brutal crackdown.
On August 8, activists and Free Syrian Army commanders distributed an 11-point code of conduct signed by scores of brigade commanders and rebel leaders.
“I swear to my people and the revolution that I will not engage in any practice that undermines the principles of our revolution: the principles of freedom, citizenship and dignity,” reads one article.
“I will respect human rights … our tolerant religious principles and international human rights law – the same human rights that we are struggling for today.”
Despite the rebels’ celebrations over the downing of the jet, regime forces have advanced into a new rebel-held district of Aleppo, the northern metropolis seen as pivotal to the outcome of the conflict.
Government forces also bombarded areas around Damascus and launched a wide sweep of the capital, including its once-bustling historic Old City.
International concern is mounting over how to end a conflict that has triggered a major humanitarian crisis and sent about 140,000 Syrians fleeing to neighbouring countries, with scores of people being killed every day.
At least 103 lost their lives on Monday: 58 civilians, 13 rebels and 32 government forces, according to the Syrian Observatory for Human Rights.
“With tanks, Syria’s regime forces have stormed the west of the district of Saif al-Dawla,” the observatory said.
“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.
In Damascus, security forces arrested 22 residents in a major security sweep in the city centre, including the Old City, while shells slammed into rebel strongholds around the capital, according to the observatory.
It was the biggest operation of its kind in the capital since the launch of the uprising, it said.
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.
The latest to flee on Monday was Syria’s top representative at the UN Human Rights Council.
Foreign ministers of Muslim states have gathered in Jeddah, with the Organisation of Islamic Cooperation expected to suspend Syria, despite divisions between host Saudi Arabia and regional rival Iran.