An earlier death toll from the strike on the rebel stronghold of Urum al-Joz reported 15 people were killed, but three more died after sustaining serious injuries.
The names of all 18 killed were listed by Ariha Today activist group, which said the death toll is likely to rise as many of the wounded remain in critical conditions.
The Britain-based Observatory said the strike was likely to have been carried out by Russian war jets, which operate in support of the Syrian regime.
The Observatory, which relies on a network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
Idlib province is controlled by a rebel alliance that includes former al-Qaeda affiliate Jabhat Fateh al-Sham, and is regularly targeted by both the Syrian regime and its Russian ally.
Earlier this week, a suspected chemical attack hit the town of Khan Sheikhun in Idlib, killing at least 87 civilians, including 31 children.
Much of the international community accused Bashar al-Assad's regime of responsibility for that attack, though Damascus involved responsibility.
In return, Moscow announced it would ramp up air defences in Syria in an attempt to show a tough line on the US actions.
Bus bombing in Homs province
Also on Saturday, Syrian state TV reported a bomb exploded aboard a bus carrying workers near the central city of Homs, killing a woman and wounding 25.
It gave no further details about the explosion, but the governor of Homs province, Talal Barrazi, told AP the bomb was placed inside a bus that transports workers at a private factory.
The Observatory confirmed a woman was killed in the blast, saying it also wounded more than 20 people.
There was no immediate claim of responsibility for the attack, but Jabhat Fateh al-Sham and the Islamic State group have carried out similar attacks in the past.
More than 320,000 people have been killed in Syria since the conflict began in March 2011 with anti-government demonstrations.
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