Syria is preventing a U.N. chemical weapons inspector from traveling to Damascus to begin the work of determining who carried out a deadly April 4 sarin attack on the town of Khan Sheikhoun, according to diplomatic sources.

A team of international experts drawn from the U.N. and the Organization for the Prohibition of Chemical Weapons issued a request on May 24 to the Syrian government to provide a visa to an Egyptian national to liaise on behalf of the inspection team with Syrian officials in Damascus.

The liaison’s job description includes arranging interviews with key officials, organizing visits to sites, and establishing contacts on both sides of the conflict. Five weeks later, the Syrian government has not responded to several follow up requests for the visa.

For months, Syria has sought to highlight its cooperation with U.N. inspectors, issuing repeated invitations to visit Khan Sheikhoun, as well as the Shayrat airbase, which Western intelligence agencies claim was used to launch the chemical weapons attack. Yet, at the same time, the regime in Damascus has stymied inspectors’ efforts to get to the bottom of who carried out the attack.

For instance, Syrian authorities have persistently ignored repeated requests from international inspectors to hand over flight logs detailing their air operations on the day of Khan Sheikhoun attack, as well as on the days of previous chlorine attacks on Syrian villages. Damascus has also ignored requests for the names of air force commanders, as well as pilots, who had responsibility for flights linked to Syria’s alleged use of chemical weapons. It has also refused to provide the inspection team with the copy of the official Syrian internal investigation into the Khan Sheikhoun attack, according to U.N. diplomats.

In a closed door session of the Security Council Thursday, Edmond Mulet, the head of the Joint Investigative Mechanism, cited those lapses while complaining that the Syrian government has not been fully cooperating with his team, according to a U.N. diplomat. Access to the Shayrat airfield is “useless unless [Syrian authorities] provide names, flight logs,” and approve the visa for a liaison officer to prepare for the team’s work, said a second diplomatic source familiar with the issue.

The revelation comes several days after a fact-finding mission by the chemical weapons watchdog concluded that the April sarin attack on Khan Sheikhoun, which killed at least 50 people, exposed nearly 300 and triggered U.S. missile strikes in retaliation, was likely caused by a chemical weapon.

Efforts to reach the Syrian mission to the United Nations Thursday were unsuccessful. But the Syrian foreign minister issued a statement Saturday, denouncing the report as lacking “any credibility.” It faulted the inspectors for relying on opposition sympathizers to provide evidence and to arrange interviews with fake victims. The testimonies provided to the inspectors, according to the statement, were “offered by terrorists in Turkey.”

Turkey has been a long time support of Syria’s armed anti-government forces.

The fact-finding mission was established in April 2014 by the OPCW’s director general, Ahmet Uzumcu, to investigate claims that chlorine had been used as a chemical weapon in attacks on several towns. The mission, which was tasked with investigating the Khan Sheikhoun attack, is limited to determining whether chemical weapons have been used, and not who used them. Continue reading.