Maaz al-Kassasbeh "was flying at a high altitude to start with. He hit the brick factory and then disappeared from sight", said Obada al-Hussein, an activist from IS-held Raqa who spoke to AFP via the Internet.
Raqa is the self-proclaimed IS "capital".
"Then the plane flew back, and this time smoke was coming out of it. I believe there was a technical failure," Hussein said.
Another activist from the city, Abu Ibrahim, also spoke of a "technical failure".
"The plane fell in an area called Hamra Ghannam, in the eastern countryside of Raqa," said Abu Ibrahim, who fled IS persecution in Raqa but who continues to be well informed on events there.
The Syrian Observatory for Human Rights gave a similar account.
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"Sources in the area saw the plane flying very low. There was a technical failure. The sources then saw IS members fire heavy machineguns and shoulder-fired missiles at the plane," said Observatory director Rami Abdel Rahman.
"The pilot ejected, after the technical problem made it impossible for him to return to a higher altitude," he added.
Nael Mustafa, another Raqa activist, told AFP: "The pilot was at a low altitude, and then IS fire hit his plane."
Their accounts came hours after the Jordanian military denied IS claims that it shot down one of its warplanes.
The US military has also dismissed the jihadists' claim to have hit the jet with an anti-aircraft missile, saying "evidence clearly suggests that ISIL (IS) did not down the aircraft".
The crash was the first warplane from the US-led coalition lost since air strikes on IS began in Syria in September, and marks a major propaganda victory for the Sunni extremist group.
Jordan is among a number of countries that have joined the air raids against IS.
Coalition warplanes have carried out regular strikes around Raqa, which IS has used as its de facto capital since declaring a "caliphate" in June straddling large parts of Iraq and Syria.