Negotiators are stalled in talks over inspection of sites where chemical weapons are stored in Syria, according to a source close to the situation.

Last month Syrian President Bashar al-Assad called on the international body to investigate an alleged breach by rebels of a storage site near the northern city of Aleppo on March 19.

But after two weeks of talks on how the probe should be carried out, no agreement has been reached, according to diplomats who spoke with reporters on condition of anonymity.

A U.N. team is standing by in Cyprus, ready to be deployed within 24 hours, U.N. Secretary-General Ban Ki-moon said last week. The team would include at least 15 members, mostly from Nordic nations, Asia and/or Latin America and none a permanent member of the U.N. Security Council.

If it is allowed to proceed, any probe would try to determine only whether chemical weapons were actually used, not the identities of who used them. 

Syria is believed to possess one of the largest arsenals of undeclared chemical weapons in the world, according to Western intelligence sources.

There have been reports of at least two other chemical warfare attacks:  one in Homs in December, and the other near the Syrian capital, Damascus, also in March.

Syrian government officials are refusing to allow inspectors to go anywhere but Aleppo. A statement by Syria’s foreign ministry on Monday said the U.N. request to go to Homs as well as Aleppo was “not in keeping with the Syrian government’s original request.”

In all three cases, opposition forces and Syrian government loyalists each blamed the other for the attacks. By the end of the month, France, Britain and the U.S. had given the U.N. information about the possible use of the chemical weapons in both places, diplomats said.

“[U.N. Secretary-General Ban Ki-moon] recognized that there is sufficient evidence to investigate both in Homs and in Aleppo,” said a senior diplomat. “They should not go in to investigate the one incident if they are told by the Syrians that they can’t investigate the second incident. So we would hope that the U.N. would not do that.”

Failing an in-person, eye-witness inspection, the U.N. could conduct its probe through eye-witness accounts told by refugees who have exited the country, according to a report by London-based television Channel 4 News.