Syrian state media say a pilot ejected Monday after suffering technical problems, while rebels claimed they shot down the government's warplane.
The official news agency SANA said a military plane encountered technical problems during what it called a "routine training mission" in eastern Syria. The pilot ejected and authorities are now searching for him.
Opposition activists posted amateur video on YouTube showing a fighter jet flying through the sound of heavy fire, erupting in a ball of flames and trailing a line of smoke. Someone off-camera celebrates, saying the plane had been hit in the town of Mohassen in Deir al-Zor province.
It is not possible to independently verify the video or the observer's claims.
According to eyewitness reports, the Syrian government has been using fighter jets as part of its most recent assaults on rebel-held areas across the country.
In the latest violence, opposition activists say 50 Syrians -- including 31 civilians -- have been confirmed dead so far Monday.
The British-based Syrian Observatory for Human Rights said most of the deaths occurred during a government bombardment on the town of Irbeen. It is impossible to independently verify the claim.
Later this week, the United Nations humanitarian chief is expected to travel to Syria and Lebanon to discuss ways of increasing aid to civilians caught up in the conflict.
Valerie Amos announced Monday that the three-day visit, beginning Tuesday, is aimed at drawing attention to the deteriorating humanitarian situation in Syria, and the impact on people who are still in the country and those who have fled to other countries, including Lebanon.
Some information for this report was provided by AP, AFP and Reuters.