The claim of rebels using sarin gas comes after months of suspicions that the Syrian regime has used the same nerve agent against rebels.
Last week, the United States said its intelligence analysts had concluded "with varying degrees of confidence" that chemical weapons had been used in Syria and that the regime of President Bashar al-Assad was the likely culprit.
In April, the head of Israeli military's intelligence research said the Syrian government is using chemical weapons against rebel forces.
"In all likelihood they used sarin gas," Brig. Gen. Itai Brun said.
The Free Syrian Army's chief of staff has also said the Syrian regime has used sarin in cities such as Homs, Aleppo and Otaiba, outside Damascus.
"We took some samples of the soil and of blood. The injured people were observed by doctors, and the samples were tested, and it was very clear that the regime used chemical weapons," Gen. Salim Idriss told CNN's Christiane Amanpour last month.
Sarin gas can be hard to detect because it is colorless, odorless and tasteless. But it can cause severe injuries to those exposed to it, including blurred vision, convulsions, paralysis and death.
Why Syria matters
The Syrian civil war has pitted rebel fighters against the regime run by al-Assad, whose family has ruled the country for four decades.
Syria matters to Iran because it is believed to be the main conduit to the Shiite militia Hezbollah in Lebanon, the proxy through which Iran can threaten Israel with an arsenal of short-range missiles.
In 2009, the top U.S. diplomat in Damascus disclosed that Syria had begun delivery of ballistic missiles to Hezbollah, according to official cables leaked to and published by WikiLeaks.
The last thing Iran wants is a Sunni-dominated Syria -- especially as the Syrian rebels' main supporters are Iran's Persian Gulf rivals: Qatar and Saudi Arabia.
Hezbollah's feared scenario is Israel on one side and a hostile Sunni-led Syria on the other.