Assad's military has significantly stepped up aerial attacks in recent weeks. Strafing from warplanes and close-range missile strikes from helicopter gunships have pushed back rebels in key fronts such as Aleppo, the country's largest city and the scene of fierce attacks to dislodge rebel positions.
The relentless bloodshed including alleged massacres by pro-regime mobs and retaliation killings by rebels has already claimed more than 20,000 lives, activists say, and will be further examined in a report expected Wednesday by the U.N. Human Rights Council's independent commission probing abuses in Syria.
In another crack in Assad's diplomatic corps, a Syrian diplomat who worked with the U.N. rights council in Geneva said he left his post to join the opposition. A spokesman for the council, Rolando Gomez, identified the Syrian as Danny al-Baaj and described him as a junior member of his country's U.N. mission. Syria is not a member of the 47-nation council, but al-Baaj worked with it as part of his duties.
The claims of bringing down the warplane and capturing the pilot, meanwhile, are likely to become a key propaganda tool to rally rebel fighters.
Activists released a video which they say showed a government Soviet-made MiG warplane catching fire after it was hit by ground fire over Deir el-Zour province, an area near the Iraqi border where the opposition has strongholds. Hours later, another video shown on the pan-Arab network Al-Arabiya purported to show the captured pilot surrounded by armed rebels. "Introduce yourself," says another speaker with his back to the camera.
The alleged captive identified himself as Col. Rafik Mohammed Suleiman and says he was on a mission to attack a rebel-held area.
"What do you tell the officers of the Assad army?" the speaker asks the man, whose beard and hair are flecked with gray. The man who identified himself as the pilot urges them to defect.
The speaker whose face remained hidden said the hostage will be treated according to tenets of Islam and the Geneva Convention. The later reference could be an indirect reply to recent international outrage over videos posted on the Internet claiming to show summary executions and torture by rebels, including bloodied prisoners being gunned down against a wall as people cried: "Free Syrian Army Forever!"
Syria's state-run SANA news agency said the pilot ejected from a warplane after a technical failure during a "training mission." It added that a search was under way to find the pilot.
The Britain-based activist group Syrian Observatory for Human Rights said the plane was hit as it was conducting air raids on the town of Muhassan, which is close to a military airfield. The group quoted activists in the area as saying the plane was hit with fire from a heavy machine gun used by rebels in the area.
Syria has such anti-aircraft weapons in its arsenal and it's possible that some could have fallen into rebel hands. In June, Syrian said it used anti-aircraft machine gun fire to bring down a Turkish F-4 Phantom fighter jet they claim crossed into Syrian aircraft. Turkey says the plane was in international flight zones.
Theodore Karasik, a regional security expert at the Dubai-based Institute for Near East and Gulf Military Analysis, said the rebel claim if true would suggest a stepped-up flow of outside military assistance. Older anti-aircraft weapons, possibly looted from Syrian arsenals such as variations on the Soviet-era SA-7, are considered overmatched by later model MiGs in the Syrian air force.
"If this is true, the conjecture would be that covert aid to the rebels is expanding with higher-grade anti-aircraft capabilities," he said.
Gulf states such as Saudi Arabia and Qatar have pledged to help supply the Syrian rebels, but the extent of the aid is unclear. Rebel commanders have consistently complained about lack of ammunition, including during recent battles in Aleppo. Last week, anti-regime protesters across Syria staged rallies calling for greater anti-aircraft firepower.
The possibility of a high-ranking military captive also could raise pressure on Assad's regime after a series of abductions, including 48 Iranians taken earlier this month and 11 Lebanese Shiites seized in May.
Rebels claim the Iranians include members of Tehran's Revolutionary Guard and were on a "reconnaissance mission" in Damascus. Iran insists the men were on a religious pilgrimage. The Lebanese are apparently held to try to pressure the government in Beirut to show greater support for the rebels which is unlikely because of the strong influence of Assad backers Hezbollah.
On Sunday, the head of Syria's main opposition group in exile called for international powers to impose a no-fly zone in border areas to protect civilians who are coming under increasingly intense attacks by regime warplanes and helicopters.
The Syrian opposition has been calling for a no-fly zone over Syria for months. But Abdelbaset Sieda, president of the Syrian National Council, renewed the plea a day after U.S. Secretary of State Hillary Rodham Clinton said Washington and Turkey were discussing a range of steps including a no-fly zone over some parts of Syria as the regime increasingly uses its air force to attack rebels.
The site of the plane incident the eastern, oil-rich region of Deir el-Zour has been the site of heavy clashes between government troops and rebels over the past week. Syrian Prime Minister Riad Hijab, who defected to Jordan earlier this month, comes from the area.
In addition to the rising death tolls in clashes more than 50 on Monday according to the activist Observatory group at least three Syria-based journalists have been killed in recent days. Al-Ikhbariya TV also said two other journalists and their driver are being held by rebels near the capital Damascus.