16 people were killed overnight in air strikes on a prison in the rebel-held city of Idlib in northwest Syria.
It said the dead included prisoners and prison guards, but did not have an immediate breakdown of the toll.
The strikes were believed to have been carried out by Russian warplanes, which have been flying sorties in support of President Bashar al-Assad's government since Moscow began a military intervention in September 2015.
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.
It added that guards opened fire in the aftermath of the strike on fleeing prisoners, and there were initial reports of additional casualties in that fire.
Idlib city became the second provincial capital to fall from government control when it was captured in March 2015 by the Army of Conquest, an alliance led by former Al-Qaeda affiliate Al-Nusra Front, later known as Fateh al-Sham.
Almost all the province remains under opposition control, and it has regularly been targeted by both Russian and government air strikes as well as raids launched by the US-led coalition fighting the Islamic State group.
Earlier this month, the US-led coalition said it was investigating allegations at least 42 people were killed in a raid it carried out in Idlib province.
More than 320,000 people have been killed in Syria since the conflict began six years ago with anti-government protests.
A cessation of hostilities was brokered by rebel backer Turkey and regime ally Russia in December, but violence has continued across much of the country.