Bashar Al-Assad
Bashar Al-AssadReuters

The White House said Wednesday it was "impossible to deny" that Syria had launched chemical weapon attacks, while calling for President Bashar Al-Assad's regime to be held accountable, AFP reported.

The comments came in response to a UN investigation which found that Assad's forces carried out at least two chemical attacks in Syria and that Islamic State jihadists used mustard gas as a weapon.

The panel was able to identify the perpetrators of three chemical attacks carried out in 2014 and 2015, but was unable to draw any conclusions in the other six cases that it has been investigating over the past year.

"It is now impossible to deny that the Syrian regime has repeatedly used industrial chlorine as a weapon against its own people," National Security Council spokesman Ned Price said in response to the report.

"The United States will work with our international partners to seek accountability through appropriate diplomatic mechanisms, including through the United Nations Security Council," he added.

Price said the findings "present yet another opportunity for all nations to speak with one voice to address these heinous crimes and to make it clear that the use of chemical weapons is intolerable."

That is likely to be seen as a veiled message to Russian leader Vladimir Putin, among others.

President Barack Obama recently warned Russia it faces fierce international rebuke if it continues to make common cause with Syria's brutal regime.

The White House is turning up the diplomatic heat as Putin prepares to travel to the G20 in China and UN General Assembly in New York in the coming weeks.

Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington, but the Organization for the Prohibition of Chemical Weapons (OPCW) has since found chlorine has been "systematically and repeatedly" used as a weapon.

Government and opposition forces in Syria have denied using chlorine and have accused each other of doing so.

The government in Damascus argues that claims that it used chemical weapons are only meant to " serve political agendas".