UNITED NATIONS • The United Nations Security Council has given the green light to an international investigation aimed at assigning blame for chemical weapons attacks in Syria, just as claims emerge that the Islamic State in Iraq and Syria (ISIS) is making crude chemical weapons.

The joint inquiry by the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW) was previously delayed due to objections by Russia, which had wanted to expand the investigation to include alleged attacks by ISIS in neighbouring Iraq.

Several diplomats said Moscow was concerned the inquiry could end up confirming allegations by Western nations and rebels that the government of Syrian President Bashar al-Assad has repeatedly used chemical weapons, including sarin and, more recently, chlorine.

Western governments hope the investigation will assign blame to individuals, who could one day be prosecuted for war crimes.

Western powers say Syria's government used chlorine, while the Syrian government and Russia accuse the rebels of using poison gas.

Syria agreed to destroy its chemical weapons in 2013 in an effort to avert US military strikes threatened over a sarin gas attack that had killed hundreds of civilians.

The OPCW has since found that chlorine has been "systematically and repeatedly" used, while a separate UN investigation has determined that sarin gas was used repeatedly in Syria.

Meanwhile, the BBC says a US government source claims ISIS is using its own crude chemical weapons in Iraq and Syria, having identified at least four occasions when ISIS used a powdered sulphur mustard agent packed into mortar rounds.

REUTERS