Also remaining to be discussed is whether any resolution would condemn al-Assad's government for the August 21 attack and whether it would call for those accused of ordering the strike to face trial before the International Criminal Court.
Both the U.N. diplomat and a Western diplomat said a vote isn't likely before mid-week, perhaps Wednesday.
The Western powers have blamed government troops for the poison gas attack, which Washington says may have killed more than 1,400 people.
Syria's government denies using chemical weapons and has said rebel forces used poison gas on its forces. But in a report issued last week, Human Rights Watch said al-Assad's forces "were almost certainly responsible," citing photos of and videos from the attack scene that pointed to weapons not known to be in rebel hands.
Opposition group wants Syrian air power restricted
A leading Syrian opposition group said Sunday that a ban on chemical weapons isn't enough, and called for the Syrian regime to end the use of air power over populated areas.
"The Syrian National Coalition insists that the ban of use of chemical weapons, which led to the loss of lives of more than 1,400 Syrian civilians, must be extended to ban the use of the regime air power and ballistic weapons against population centers, in addition to the redeployment of heavy weapons away from population centers, and the prohibition of use of these weapons to bomb Syrian cities and villages," the group said in a statement.
"The world must not allow the Assad regime to take advantage of the Russian initiative and their joining the treaty on the prohibition of the use of chemical weapons as an excuse to continue the daily slaughter of the Syrian people with impunity," it added.
The group expressed skepticism about the chemical weapons deal.
"The Assad regime has a long track record of deceit when it comes to dealing with the treaties and empty promises in the regime attempt to buy more time," the coalition statement said. It called on its supporters in the region and in the West to strengthen its military capabilities.
"The coalition calls upon the Arab brothers and Friends of Syria military to strengthen the arm capability of the military opposition to be able to neutralize the Assad air force and his tanks to force the regime to end its military campaign and accept a political solution that will lead to a democratic transition in Syria," it said.
Last week, Gen. Salim Idriss, head of the rebel Free Syrian Army, said he has information that Syria has already started to move chemical materials and weapons out of the country, into Lebanon and Iraq. Iraq denied the allegation, calling it "cheap propaganda."
"We confirm that Iraq is against possessing such weapons and other weapons of mass destruction anywhere in the world and under any excuse," said a statement Sunday from the Iraqi prime minister's office.