Syrian President Bashar al-Assad may be preparing a chemical weapons attack that would result in the "mass murder" of civilians, the White House said Monday, warning the regime would pay a "heavy price" if it goes ahead.

"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."