It said the death toll exceeds 50, noting that more bodies are being discovered.
The Britain-based watchdog group said the US-led coalition targeted the meeting with at least seven highly explosive missiles.
The US Central Command confirmed that it had attacked leaders of al-Qaida in Idlib province, noting that the operation had targeted those "responsible for attacks threatening U.S. citizens, our partners, and innocent civilians."
The US airstrike comes as a cease-fire was declared by the Syrian army on Friday and went into effect on Saturday.
The new cease-fire aims at halting the fighting in Idlib between the Syrian army and the al-Qaida-linked groups.