On Wednesday, Syrian Free Army leader Gen. Salim Idriss told CNN's Christiane Amanpour that the Syrian government has used chemical weapons in several places, including Homs, Aleppo and Otaiba, near Damascus.
Idriss said rebel forces had some of the people reportedly exposed to chemical weapons examined by doctors, and they took soil and blood samples.
"And the samples were tested, it was very clear that the regime used chemical weapons," he said.
The British Foreign Office said Thursday that it had "limited but persuasive" evidence of chemical weapons use in Syria, and it urged al-Assad to allow unfettered access for international investigators.
Ventrell told reporters on Thursday that allowing a team of U.N. experts to investigate would be the "most direct way" to find out what happened.
"We have the U.N. ready to deploy inspectors on a couple of days notice. We're already pre-deploying out in the field near Syria and ready to go in," he said. "And you have the regime resisting a full and thorough and credible investigation within Syria. So we urge the regime to allow these inspectors in. That currently is the most direct route to getting to the bottom of what may have occurred."
NATO leaders discuss the issue
The administration disclosure comes two days after Secretary of State John Kerry urged NATO members to prepare for the possible use of chemical weapons by the Syrian regime, while Russia's foreign minister accused the West of politicizing the search for such weapons, comparing it to the hunt for weapons of mass destruction in Iraq.
Russian Foreign Minister Sergei Lavrov warned Tuesday that reports of chemical weapons in Syria must be carefully investigated to avoid a repetition of the "Iraqi scenario" in which unconfirmed allegations that the regime of Saddam Hussein possessed weapons of mass destruction were the basis for the U.S.-led invasion.
He accused Western nations of trying to "politicize the issue" and broadening the investigation. Experts were supposed to be sent to Syria to study the possible use of chemical weapons in Aleppo. Instead, Lavrov said, investigators demanded access to all facilities in Syria and the right to interview all Syrian citizens.
"I believe that is too much," he said.
NATO remains conflicted about Syria's two-year civil war. While members are concerned about the mounting causalities, millions of refugees and the potential for a wider regional spillover, they are loath to become embroiled in another Middle Eastern conflict.
Earlier this week an Israeli intelligence official said Damascus was using weapons banned under international law against its own people in the country's civil war.
On Wednesday, Israeli President Shimon Peres said that he expected the United States to fall in line with its estimate on chemical weapons use in Syria.
"I think the United States and us and others will do whatever we can to meet this very dangerous weapon," he said. "The sooner the better."
Syria has said it is rebels who have used chemical weapons.
Sarin gas is an odorless nerve agent that can cause convulsions, paralysis and respiratory failure. Nerve agents are the most toxic and rapidly acting of the known chemical warfare agents, according to the Centers for Disease Control and Prevention.
Sarin quickly evaporates from liquid to vapor form to disperse into the environment. It also mixes easily with water and can poison a water supply.