The activists said the airstrike late yesterday in Boukamal, on the Iraqi border, was similar to those carried out by the US-led coalition that has been targeting the Islamic State group in both countries.
Airstrikes by the US-led coalition have killed dozens of civilians over the past several weeks as the battle against the extremists intensifies.
Human Rights Watch said in a report today that US forces appear to have failed to take necessary precautions to avoid civilian casualties in a March airstrike that killed dozens of people in the northern Syrian province of Aleppo.
Syrian opposition activists said around 40 people, mostly civilians, were killed in the crowded Omar Ibn al-Khattab Mosque in the Jeeneh district. US officials at the time said they found no basis for reports that civilians were killed.
More From This Section
Human Rights Watch's 16-page report said it found no evidence to support the allegation that members of al-Qaida or any other armed group were meeting in the mosque. It said a religious lecture was being held at the time of the attack and prayers were about to begin.
"The US authorities need to figure out what went wrong, start doing their homework before they launch attacks, and make sure it doesn't happen again."
Opposition groups said yesterday's strike on Boukamal killed and wounded dozens.
The Britain-based Syrian Observatory for Human Rights said the airstrike killed 13 civilians, including women and children, as well as three Iraqi IS fighters.
Opposition activist Omar Abu Laila, who currently lives in Europe but is from the province where the strike took place and maintains contacts there, said 10 people were killed, including an Iraqi family of four. He said IS cordoned off the area and cut all telecommunications after the strike, which heavily damaged at least four buildings.
IS used to control wide parts of Syria and Iraq, where the group declared a caliphate in 2014. Over the past year the extremists have suffered a string of defeats at the hands of an array of Syrian and Iraqi forces.