Syria is charging in a letter to the United Nations that opposition groups are planning a toxic gas attack in a rebel-held area near Damascus so they can then blame it on government security forces.

In a letter dated March 25 and circulated by the U.N this week, Syria's U.N envoy, Bashar Ja'afari, said his government had intercepted communications between "terrorists" that showed a man named Abu Nadir was secretly distributing gas masks in the rebel-held Jobar area.

"The authorities also intercepted another communication between two other terrorists, one of whom is named Abu Jihad," Ja'afari said. "In that communication, Abu Jihad indicates that toxic gas will be used and asked those who are working with him to supply protective masks."

Ja'afari said in the letter addressed to U.N Secretary-General Ban Ki-moon and the U.N. Security Council that this information "confirms that armed terrorist groups are preparing to use toxic gas in Jobar quarter and other areas, in order to accuse the Syrian government of having committed such an act of terrorism."

A senior Western diplomat, speaking on condition of anonymity, said of the Syrian intelligence: "I don't give any credence to that." A United Nations inquiry found in December that sarin gas had likely been used in Jobar in August and also in several other locations, including in the rebel-held Damascus suburb of Ghouta, where hundreds of people were killed.

The inquiry was only looking at whether chemical weapons were used, not who used them. The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied it.

Syrian President Bashar al-Assad agreed to destroy his chemical weapons following global outrage over the large-scale sarin gas attack in Ghouta in August. The gas attack sparked a U.S. threat of military strikes, which was dropped after Assad's pledge to give up chemical arms.

But the Syrian government, locked in a three-year-old war with rebels seeking to overthrow Assad, failed to meet a February 5 deadline to move all of its declared chemical substances and precursors, some 1,300 tonnes, out of the country.

Syria has since agreed to a new timetable to remove its chemical weapons by late April. Sigrid Kaag, head of the joint Organization for the Prohibition of Chemical Weapons and United Nations mission overseeing the removal of Syria's chemical weapons, is due to brief the Security Council today.

Kaag said last month that Syria could ship out its remaining chemical weapons within a month and still meet a mid-year target for their final destruction. Syria is responsible for transporting the chemicals to its Mediterranean port of Latakia, where they will be shipped abroad for destruction.

In a separate letter to Ban and the Security Council, Syria's Ja'afari also warned that "armed terrorist groups continue to threaten and carry out terrorist attacks against chemical weapons facilities and the chemical substances."

The senior Western diplomat said: "I don't think there's any evidence that any of the groups have any interest in attacking the convoys ... we don't see that as a major risk." Syria's three-year civil war has killed more than 150,000 people, a third of them civilians, and caused millions to flee.