US Defence Secretary James Mattis has said that Syria's chemical weapons programme goes "far beyond one airfield".

The comment appears to be a reference to the Syrian airfield which the United States struck in April, alleging the President Bashar al-Assad had used it to launch a chemical weapons attack on his own people.

Mr Mattis said he believes the Syrian regime has now stockpiled chemical weapons in other parts of the country, in violation of international agreements.

The comment comes days after the White House issued a warning to the Syrian regime that Mr Assad would "pay a heavy price" for any further attacks. Mr Mattis said he beliefs the warning had its intended effect.

"It appears that they took the warning seriously," Mr Mattis told reporters. "They didn't do it."

Syria was said to have destroyed all of their chemical weapons under an agreement reached by the US, Russia, and the Assad regime in 2013. But in April, more than 80 people were killed in a suspected chemical weapons attack in north-western Syria.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

The White House said on Monday that it had identified potential preparations for another chemical weapons attack by Mr Assad's regime.

"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria," the White House 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."

A Pentagon spokesperson later confirmed the alleged preparations were being made at the same airfield the US struck in April. That strike marked the first direct military action taken by the US against Mr Assad's regime since the beginning of the country's six-year civil war. Six people were reportedly killed.

Secretary of State Rex Tillerson said the strike did not signal a policy change toward Syria, but proved Mr Trump is "willing to act when governments and actors cross the line ... and cross the line in the most heinous of ways".

Mr Assad has claimed innocence in the chemical weapons attack, and called the airstrike a "disgraceful act" that "can only be described as short-sighted."

US forces have since shot down several of the Syrian regime’s drones, which they claim fired on – or came close to firing on – a US training camp in the country. The US has also launched two direct attacks on Syrian forces entering a deconfliction zone they established around the camp.

The US maintains that their presence in the country is part of a fight against Isis, not to undermine the Syrian regime.