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.

To address that gap, the U.N. Security Council in August 2015, established a second investigative team - the Joint Investigation Mechanism- to assign blame. It is now up to that team, which is seeking a visa for a liaison official in Damascus, to determine who attacked Khan Sheikhoun.

Syria has repeatedly appealed to international weapons inspectors to come and examine their claim that the rebels did were responsible for the April 4 chemical weapons attack on Khan Sheikhoun. The offer, according to Syrian authorities and their Russian and Iranian backers, demonstrates the government's confidence that the evidence will support its case.

The OPCW has turned down the offer, insisting the it is too dangerous to travel to Khan Sheikhoun. The watchdog organization has also declined an invitation to visit the Shayrat airbase on the grounds that Syria can furnish the information they need - flight logs, and air force personnel- without having to travel to Shayrat.

The fact-finding mission's account, meanwhile, undercut a claim by the Syrian government that civilians in the opposition-controlled town were exposed to sarin gas after a Syrian war plane bombed a suspected jihadist compound that was storing chemical agents. The investigators concluded the release of sarin or a sarin-like substance "was most likely initiated at the site where there is now a crater in the road."

Ironically, a critical bit of supporting evidence - soil samples collected by an unnamed volunteer - was provided to the international inspectors by the Syrian government, and tested at one of its own labs. The strongest concentrations of sarin, as well as hexamine, an acid scavenger that has been used by Syria chemical weapons program to stabilise sarin, were detected in soil samples from inside the crater.

Those findings also contrasted with the accounts of two eyewitness put forward by the Syrian government. The two individuals, who were interviewed on June 21 and 22, claimed that the toxic gas was likely caused by the bombing of an opposition weapons storehouse.

Two months before the sarin incident, according to one interviewee, an armed opposition group had evicted locals from a home and used it to store weapons, munitions and barrels. The building, according to the source, "appeared to have been damaged on April 4."

A second interviewee claimed that he was awoken on April 4 "by the sounds of an explosion and observed a cloud above a building which he described as a 'chemical warehouse.'"

It was too dangerous for the inspection team to travel to Khan Sheikhoun, which is under opposition control, in the crucial weeks after the sarin attack, complicating efforts to secure useful evidence.

For instance, the fact-finding team has not been able to establish what kind of munition was used in the attack, a vital piece of evidence that could help establish the culprit. It has also been forced to rely on evidence collected by Syrian government or local advocacy groups, and blood, urine and hair samples collected from victims who fled to an unidentified neighboring country for medical treatment.

The investigators fashioned a competing narrative indicating a chemical weapon was the most likely source of the sarin exposure.

According to this account, which was based on interviews with 34 victims, doctors and eyewitnesses, a jet dove with a "swooping sound" over Khan Sheikhoun in the early morning hours. But in contrast to other bomb attacks, there was no loud explosive.

A witness told the team that he had seen a young boy who sells fuel in the town walking away from the bomb site and then falling. The witness passed out after he came to the boy's aid, awakening later in the hospital.

The investigators observed the autopsies of three victims, and took blood samples taken from vital organs, including the brain, liver and lungs, showing evidence of sarin or a sarin-like substance. The also team obtained the blood samples from eight other victims who tested positive for the toxic agent.

Those samples, the investigators found, "provide incontrovertible evidence that people were exposed to a sarin or a sarin-like substance."