He said the requests have come from all areas of Syria, neighbouring countries and even globally.
However, the OPCW is not mandated to fix blame for the attack.
The exercise is part of an ongoing investigation into the suspected chemical attack by the Syrian regime that killed dozens in the enclave on April 7.
Husari told The Associated Press on Wednesday that his production, which is still a work in progress, is a fictional short drama based on the chemical weapons attack in Ghouta that took place in 2013. The U.S. President Donald Trump on April 13 ordered missile attacks at the territory of Syria as a reaction on chemical weapon use in Douma.
The Red Cross head said that the surge was probably in part linked to Syrians returning to "precariously stable" areas where combat had faded and that requests came from Syrians displaced inside Syria and overseas. Their entry into the town was delayed by security fears.
Meanwhile, global donors on Wednesday pledged $4.4 billion in aid for war-hit civilians of Syria.
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The Russian delegation to the OPCW will hold a briefing on Thursday in The Hague at which it will present "some Syrians to speak about the reported Douma incident", the agency's statement said.
Russia's Defense Ministry has also accused Britain of direct involvement in staging video images of alleged victims.
The OPCW does not assign blame for chemical attacks and there is now no mechanism that can do so after Russian Federation vetoed a resolution to extend the mandate of a United Nations-OPCW mission in November. Britain vehemently denied the Russian accusation.
The Syrian air defense was able to successfully shot down 46 cruise missiles that the US, France and the United Kingdom fired at it on April 14, a senior Russian military commander says, rejecting American claims that all of the missiles hit their targets.
In the days since the April 14 Western air strikes, top Russian officials have said they may reconsider a pledge Moscow gave a decade ago not to provide Syria with the S-300 system. Previous reports had said Moscow could supply Damascus with the S-300 system, a development that has troubled Israeli officials as the surface-to-air missile can lock onto targets deep inside Israeli airspace. Josh T. Jacques responded to Russian Federation sending in additional missile defenses, telling Business Insider that Russian Federation "should move humanitarian aid into Syria, not more weaponry".
With fighting still raging and the northern rebel-held province of Idlib in danger of becoming a new crisis zone, European Union foreign policy chief Federica Mogherini urged Russia, Iran and Turkey to use their influence in Damascus, saying they have a "special responsibility" to establish a cease-fire and to press Assad to return to the negotiating table.