The Observatory`s head said the planes could be Russian or Syrian but were not from the US-led coalition.
Rami Abdel Rahman told AFP charred bodies of fighters were at the scene.
The Observatory, which monitors the conflict in Syria using a network of sources on the ground, said the convoy was travelling from the IS de facto capital of Raqqa when it was attacked.
Russia began its air strikes over Syria at the end of last month, saying they were targeting IS and other militant groups opposed by the regime of President Bashar al-Assad.
The US-led coalition, which has been carrying out its own air strikes against IS - has said it will not be cooperating with Russia`s mission.
Anti-government protests developed into a civil war that four years on has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
What`s the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
Iran, Russia and Lebanon`s Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran have pro-Assad forces on the ground, while a Western-led coalition and Russia are carrying out air strikes.
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