BEIRUT, Lebanon — Last month’s chemical weapons attack on a rebel-held Syrian town may have caught the world’s — and President Trump’s — attention, but it was not the only recent suspected use of a nerve agent by Syrian government forces.

On three other occasions in the months leading up to the attack on the town of Khan Sheikhoun, witnesses, doctors and human rights investigators say, government attacks left scores of people sickened with similar symptoms, like foaming at the mouth, shaking and paralysis — including two attacks in December, little noticed at the time, that killed at least 64 people.

New information about the additional attacks appears in a Human Rights Watch report released Monday, bolstering New York Times reporting on those episodes and placing Khan Sheikhoun in the context of wider evidence that the Syrian government continues to use chemical weapons despite its 2013 agreement to give them up.

Despite the missile strike Mr. Trump ordered on the Syrian military airfield he said was the source of the Khan Sheikhoun attack, Syrian forces are doubling down on tactics that constitute war crimes, including bombing hospitals and rescue and medical workers and using chemical weapons, according to the report and other witness accounts.