Diplomats say the West has firm proof that chemical weapons have been deployed at least once over the course of the civil war that has been ravaging Syria for two years.

“In one case, we have hard evidence,” one diplomat was quoted by AFP as saying Thursday. And “there are several examples where we are quite sure that shells with chemicals have been used in a very sporadic way.”

Syria is believed to possess one of the world’s largest stockpiles of weaponized chemical agents.

According to the diplomat, who spoke on condition of anonymity, UN investigators would likely not be given leave to conduct an investigation on Syrian soil, after the country’s embattled president, Bashar al-Assad, stonewalled the probe.

On Monday, UN Secretary-General Ban Ki-moon urged the Syrian government to accept an expanded UN inquiry into alleged chemical weapons use, saying he had concluded that an alleged attack in Homs in December warranted investigation.

Syria on Monday rejected the expanded investigation, which it had originally sought to look into alleged use of chemical weapons by rebels in March on the village of Khan al-Assal. The rebels blamed regime forces for that attack.

Britain and France asked the UN to investigate allegations of chemical weapons use in Khan al-Assal and another village, Ataybah, on March 19, as well as Homs on December 23.

The March 19 attack killed 30 and left another 80 injured.

Speaking Tuesday in Rome, Ban said he had decided the mission should also look into the Homs allegations, and urged Syria to “extend its fullest cooperation and allow the investigation to proceed.”

US President Barack Obama said in 2012 that the use of chemical weapons by the Syrian government would cross a “red line” and prompt direct action from the United States.

The US, Israel, and Jordan are particularly concerned about the fate of Syria’s stockpile of chemical weapons, which is believed to be divided among several storage sites throughout the country.

In addition to the possibility that Assad may use the weapons as a desperate measure to stay in power, there are fears that some of the weapons may find their way into the hands of terror groups in Syria or Lebanon.

The Associated Press contributed to this report.