BEIRUT, Lebanon — One of the worst chemical bombings in Syria turned a northern rebel-held area into a toxic kill zone on Tuesday, inciting international outrage over the ever-increasing government impunity shown in the country’s six-year war.

Western leaders including President Trump blamed the Syrian government of President Bashar al-Assad and called on its patrons, Russia and Iran, to prevent a recurrence of what many described as a war crime.

Dozens of people, including children, died — some writhing, choking, gasping or foaming at the mouth — after breathing in poison that possibly contained a nerve agent or other banned chemicals, according to witnesses, doctors and rescue workers. They said the toxic substance spread after warplanes dropped bombs in the early morning hours. Some rescue workers grew ill and collapsed from proximity to the dead.

The opposition-run Health Department in Idlib Province, where the attack took place, said 69 people had died, providing a list of their names. The dead were still being identified, and some humanitarian groups said as many as 100 had died.

The government of Mr. Assad, who renounced chemical weapons nearly four years ago after a large chemical attack that American intelligence agencies concluded was carried out by his forces, denied that his military had been responsible, as he has done every time chemical munitions have been used in Syria.

A statement from the Syrian military accused insurgents of responsibility and said they had accused the army of using toxic weapons “every time they fail to achieve the goals of their sponsors.”