The resolution calls on the UN's Organisation for the Prohibition of Chemical Weapons (OPCW) to report quickly on its fact-finding mission on the attack that killed at least 70 civilians.
Twenty children were among the dead, according to the London-based Syrian Observatory of Human Rights, and dozens more civilians suffered respiratory problems and other symptoms such as vomiting, fainting and foaming at the mouth.
At least another 100 people were being treated in hospitals in Syria's Idlib province and dozens of others, some in a critical condition, have been transferred to Turkey, according to press reports.
The draft resolution "condemns in the strongest terms the use of chemical weapons" in Syria, in particular the attack on Khan Sheikhun and expresses "outrage" over the use of toxic gases in Syria's six-year civil war.
It calls on President Bashar al-Assad's government to provide flight plans, flight logs and other information on its military operations on the day of the assault.
The draft also wants Damascus to provide the names of all commanders of helicopter squadrons to UN investigators, allow them to meet generals and other high-ranking officials within five days of their request and visit air bases from which the attacks could have been launched.
"The perpetrators must be held accountable. We need to address this issue at the Security Council, as soon as possible," said French deputy UN ambassador Alexis Lamek.
The Syrian military has denied any involvement but UK Foreign Secretary Boris Johnson said "All the evidence I have seen suggests this was the Assad regime... using illegal weapons on their own people," when arriving at a Syrian aid conference in Brussels on Wednesday.
"The horrific events of yesterday demonstrate unfortunately that war crimes are going on in Syria [and] international humanitarian law is being violated frequently," UN chief Antonio Guterres said on Tuesday.
Russia claims terrorist warehouse hit
Russia on Wednesday said the air strike hit a "terrorist warehouse" containing "toxic substances".
"The arsenal of chemical weapons" was destined for fighters in Iraq, a defence ministry statement said, adding that the information was "completely reliable and objective."
The use of such weapons "by terrorists has been repeatedly proved by international organisations as well as official authorities" in Iraq.
In February Russia and China vetoed a proposed Security Council resolution that would have imposed sanctions on Syrians accused of involvement in chlorine gas attacks on villages in 2014 and 2015.