The United States has blacklisted 271 employees of a Syrian government agency it said was responsible for developing chemical weapons, weeks after a poison gas attack killed scores of people in a rebel-held province in Syria.

The US Treasury Department on Monday said the employees of Syria's Scientific Studies and Research Centre are experts in chemistry and related fields, or have worked in support of the centre's "chemical weapons programme" since at least 2012, or both.

"These sweeping sanctions target the scientific support centre for Syrian dictator Bashar al-Assad's horrific chemical weapons attack on innocent civilian men, women, and children," US Treasury Secretary Steven Mnuchin said in a statement.

The Syrian government said its military "did not and will not" use chemical weapons, denying accusations that it was behind an attack on a rebel-held town of Khan Sheikhoun in Idlib province that sparked widespread international outrage.

READ MORE: Syria denies using chemical weapons in Idlib

At least 86 people were killed and hundreds were treated in hospital after the chemical attack in Khan Sheikhoun, according to the Syrian Observatory for Human Rights, a UK-based monitoring group.

The deaths, blamed by many on Assad's government, prompted the United Nations to pledge it would investigate the incident as a possible war crime.