A Syrian warplane was shot down Tuesday by "terrorist organizations" using a surface-to-air missile, state news agency SANA said.
Hardline Islamist rebel group Ahrar al-Sham claimed responsibility for the downing on its official Twitter account.
The Syrian Observatory for Human Rights, a monitoring group, said the Sukhoi Su-22 fighter-bomber was shot down by Syrian al-Qaeda affiliate the al-Nusra Front.
The incident is the second reported downing of a Syrian plane with anti-air weaponry, after a similar incident last month.
The incident, in particular the reported role of the Syrian branch of al-Qaeda, is likely to cause concern to Western backers of Syrian rebels.
Despite repeated pleas from Syrian rebels for assistance against the regime air force, the United States has largely rejected the transfer of anti-aircraft weapons to the opposition for fear they would fall into al-Qaeda hands.
Analyst Charles Lister of the Washingon-based Middle East Institute, writing on Twitter, said that portable anti-air systems had been transferred to Syrian rebels on several occasions "to be used for select political purposes."
The Observatory, which compiles reports from a network of activists on the ground across Syria, said that the pilot was captured by al-Nusra Front fighters south of Aleppo. The fate of the co-pilot was still unknown, it said.
Al-Qaeda and other rebel groups have gained ground from pro-government forces in the area in recent days.
Lister wrote that the rebel advance represented "a victory for al-Qaeda in its efforts to undermine the political process."
The failure of al-Assad's Russian ally to intervene with its air power to beat back the attack might point to "a potential source of pressure" that Moscow could use to ensure al-Assad complies with the timetable for UN-brokered peace talks, he added.
A UN spokesman in Geneva Tuesday told reporters that the next round of talks would start on Monday.
Fighting on the ground also continued around Aleppo, despite a partial ceasefire which went into effect in late February.
Ten civilians were killed when rebels shelled the Kurdish-held Aleppo district of Sheikh Maqsoud, the Observatory said.