The United States has warned the Assad regime in Syria may be making preparations for another chemical weapons attack on civilians.

In a statement from the Press Secretary today, the United States said it "has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children".

The statement warned Bashar al-Assad and his military "will pay a heavy price" if he conducts another attack like the one on April 4, 2017.

Bashar al-Assad.

"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria," spokesman Sean Spicer said in a statement.

"If, however, Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."

French intelligence determined in late April that the Bashar al-Assad government authorised the deadly nerve gas attack in northern Syria on April 4.

Scores of people were killed in the chemical weapons attack on the town of Khan Sheikhoun.

"We know, from a certain source, that the process of fabrication of the samples taken is typical of the method developed in Syrian laboratories," Foreign Minister Jean-Marc Ayrault told reporters at the time.

"This method is the signature of the regime and it is what enables us to establish the responsibility of the attack."

In two interviews since April 4, the Syrian President has said evidence of a poison gas attack was false and denied his government had ever used chemical weapons.