Syria's government and opposition have called for an inquiry into an attack that seems to have involved chemical weapons, both sides blaming each other.
Syria's UN envoy said he had asked the UN secretary general to set up an inquiry into Tuesday's attack.
Opposition leaders meanwhile demanded a "full international investigation".
US President Barack Obama has said he is "deeply sceptical" of the Syrian government's allegations that rebel forces used such weapons.
Mr Obama, on a visit to Israel, said some Syrian government officials had "expressed a willingness to use chemical weapons if necessary to protect themselves".
He repeated his insistence that chemical weapons were a "red line" for the US in terms of its involvement in Syria, saying using them against Syrian people "would be a serious and tragic mistake".
Earlier, US ambassador Robert Ford said there was no evidence to support claims that a rocket containing poisonous gas hit Khan al-Assal, killing some 20 people.
Mr Ford, who left Damascus more than a year ago, warned that there would be "consequences" for the Syrian government if it was found to have used chemical weapons for the first time since the start of the uprising against President Bashar al-Assad.
The first report of the alleged chemical weapon attack in the Khan al-Assal area of Aleppo province came from Syria's state news agency Sana.
The report said "terrorists" had fired a rocket "containing chemical materials", killing 16 people.
Sana published photographs of what it claimed were victims, although they appeared to show no signs of exposure to chemical weapons.
The official death toll rose to 31.
Later, rebel commanders accused government forces of carrying out the attack, and cited reports of victims suffering breathing difficulties and bluish skin.
One commander said warplanes had mistakenly bombed a government-controlled area.
On Wednesday, the main opposition alliance, the National Coalition, said in a statement that 19 people had been killed and that many victims had suffocated.
Neither of the two sides' claims could be verified independently.
The National Coalition's statement also called for a "full international investigation" and asked for a delegation to be sent to Khan al-Assal as soon as possible.
"All evidence now indicates that the Assad regime is using these weapons against its own people. Testimonies and images from the attacks today demonstrate that these banned weapons have been used in what amounts to a crime against humanity."
"The coalition would like all parties and individuals involved in this reprehensible crime to be brought to justice," the statement added.
Syria's permanent representative to the UN, Bashar Jaafari, meanwhile told reporters in New York that he had asked Secretary General Ban Ki-moon to form a "specialised, independent and neutral technical mission to investigate the use of the terrorist groups operating in Syria of chemical weapons yesterday against civilians".
Mr Jaafari called the attack "very serious and alarming, and unacceptable and unethical", and said the UN's help had been sought as "a sign of good faith, good will, good intentions" to the international community, international public and the Syrian people.
Foreign Minister Walid Muallem accused European countries and the Arab League of encouraging the rebels to "commit their odious crime" and reiterated that the Syrian government would "not use chemical weapons, if they exist, against its own people".
UN spokesman Martin Nesirky said the secretary general remained convinced that the use of chemical weapons by any party under any circumstances would constitute "an outrageous crime".