The Syrian army "categorically denies the use of chemical and poisonous material in Khan Sheikhoun", the General Command said in a statement.
The incident brought swift global condemnation, with France and Britain demanding an emergency UN Security Council meeting and President Francois Hollande denouncing a "massacre".
A joint investigation by the United Nations and the worldwide chemical weapons watchdog determined the Syrian government was behind at least three attacks in 2014 and 2015 involving chlorine gas and that the Islamic State group was responsible for at least one, involving mustard gas.
Syria's peace talks got off to a rocky start yesterday when rebel groups refused to meet President Bashar al-Assad's government and a war of words broke out between the two sides.
If Tuesday's reports of a chemical attack by the regime are confirmed, experts say it could be the biggest since Ghouta.
British U.N. Ambassador Matthew Rycroft said the Idlib attack "demonstrates once again that the regime will stop at nothing to remain in power, even the use of the most heinous weapons imaginable".
Syrian opposition officials maintain that the government's stocks were not fully accounted for, and that it retained supplies.
The town is in Syria's Idlib province, which is largely controlled by an alliance of rebels including former Al-Qaeda affiliate Fateh al-Sham Front.
The White House condemned what it said was a "reprehensible" attack carried out by Assad s forces. The High Negotiations Committee, an umbrella opposition group, claimed the death toll could be as high as 100.
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"Most of the hospitals in Idlib province are now overflowing with wounded people", Khalil told a news conference in Idlib.
Horrific reports of chemical weapons attack in Idlib, Syria.
An investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Islamic State militants had used mustard gas.
A joint UN-OPCW report published last October said government forces used chlorine in a toxic gas attack in Qmenas in Idlib province in March 2015. One video showed a victim with constricted pupils, a symptom experts said was consistent with the use of a nerve agent or other chemical weapon.
The European Union's top diplomat Federica Mogherini said: "Obviously there is a primary responsibility from the regime because it has the primary responsibility of protecting its people".
Mohammad Alloush, head of the rebel delegation, said he was "ready to go to the ends of the earth" to end six years of bloodshed, which has seen 300,000 people killed and 11 million displaced, but the continued attacks on rebel areas near Damascus meant he could not take part in the first round of direct talks.
However, the airstrike that took place this morning in Idlib claiming dozens of lives utilized sarin nerve gas: a lethal gas that has been defined by the United Nations as a weapon of mass destruction and has been prohibited for use and collection by the Chemical Weapons Convention.
August 7, 2015: The U.N. Security Council authorizes the OPCW and U.N. investigators to probe reports of chemical weapons use in Syria, as reports circulate of repeated chlorine gas attacks by government forces against civilians in opposition-held areas.