UNITED NATIONS - The United States, Britain and France vowed on the fifth anniversary of a chemical weapons attack that they blame on the Syrian government to take action as they have in the past against any further attacks by President Bashar Assad's regime.

A joint statement issued late Tuesday called the Aug. 21, 2012 sarin nerve gas attack that killed hundreds of people in the Ghouta suburb of Damascus "horrific.'' The use of sarin led to a U.S.-Russian agreement to eliminate Assad's chemical weapons, which averted U.S. military strikes against Syria.

Since then, the three Western powers have accused Syria of resorting to the use of chemical weapons during military offensives in Khan Sheikhoun, Ltamenah, Saraqeb and Douma. Following the suspected chemical attack in Douma in April, the U.S., U.K., and France launched punitive military strikes in Syria.

"As we have demonstrated, we will respond appropriately to any further use of chemical weapons by the Syrian regime, which has had such devastating humanitarian consequences for the Syrian population,'' the statement said.

The three governments implored Assad's supporters "to recognize that the unchecked use of chemical weapons by any state presents an unacceptable security threat to all states.''

The U.S., U.K. and France also expressed grave concern at reports of a Syrian government military offensive against civilians, schools, hospitals and other civilian infrastructure in the northern province of Idlib, the last major rebel-held bastion, and underlined "our concern at the potential for further - and illegal - use of chemical weapons.''

"We remain resolved to act of the Assad regime uses chemical weapons again,'' the Western allies warned.

The 2012 U.S.-Russia agreement required Syria to join the Organization for the Prohibition of Chemical Weapons and declare all its chemical weapons and precursors.

Syrian President Bashar Assad said in an interview in June with Russia's state-controlled NTV television channel that his government got rid of all its chemical weapons in 2013 and that allegations of their use are a pretext for invasion by other countries.

But there is growing frustration at Damascus' failure to satisfactorily answer all outstanding questions from the OPCW about its declaration.

U.N. Secretary-General Antonio Guterres reiterated in a letter transmitting the latest OPCW report to the Security Council that all open issues in the declaration must be resolved, and he strongly encouraged the Syrian government to do so.

The U.S., U.K., and France welcomed the June 27 decision by OPCW member nations to take over the responsibility for determining blame for chemical attacks, saying this "will help ensure that the perpetrators of chemical weapons use in Syria cannot escape identification.''

The Security Council established a joint U.N.-OPCW investigative team in August 2015 to determine responsibility for chemical attacks in Syria.

The so-called Joint Investigative Mechanism known as the JIM accused Syria of using chlorine gas in at least two attacks in 2014 and 2015 and the nerve agent sarin in an aerial attack on Khan Sheikhoun in April 2017 that killed about 100 people and affected about 200 others. The Khan Sheikhoun attack led to a U.S. airstrike on a Syrian airfield.

The JIM also accused the Islamic State extremist group of using mustard gas twice in 2015 and 2016.

Russia, a close ally of the Assad government, vetoed a Western-backed resolution last November that would have renewed the JIM mandate, leaving no way to determine accountability for chemical attacks in Syria.

A Western-led campaign that included the U.S., U.K. and France succeeded in expanding the OPCW's investigations, which were limited to determining if chemical weapons were used in Syria, so that it can now determine responsibility for attacks as well.