The Syrian government says its military "did not and will not" use chemical weapons, denying accusations it was behind an attack on a rebel-held town in Idlib province that sparked widespread international outrage.

At least 86 people were killed and hundreds - including people foaming at the mouth - were hospitalised after Tuesday's chemical incident on the town of Khan Sheikhoun, according to the Syrian Observatory for Human Rights, a UK-based monitoring group.

The deaths, blamed by many on the government of Syrian President Bashar al-Assad, prompted the United Nations to pledge it would investigate the incident as a possible war crime.

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But Walid Muallem, Syria's foreign minister, on Thursday denied the charges, calling them an unjust campaign against Syria launched by countries known for their hostility.

"I stress to you once again: the Syrian army has not, did not, and will not use this kind of weapons - not just against our own people, but even against the terrorists that are targeting our civilians indiscriminately," he said at a press conference in Damascus.

Muallem said the Syrian army had bombed a warehouse belonging to rebels that contained chemical weapons, echoing earlier comments made by Russia, a key ally of the Assad government. The rebels deny this.

"The first air raid conducted by the Syrian army was at 1130am (0830:GMT) on that day [Tuesday] and it attacked an arms depot belonging to al-Nusra Front [al-Qaeda's former Syria affiliate] chemical weapons," he said.

Responding to calls for an international investigation, Muallem said the Syrian government needed guarantees that any fact-finding mission into Idlib's attack would be impartial and not "politicised".

The foreign minister said Syria's experience with past missions is "not encouraging", adding that Damascus would only consider the idea of an inquiry if its concerns were addressed.

"He said that he believes that results will be changed, even if they allow full access to any international investigators," Al Jazeera's Alan Fisher, reporting from Beirut, said.

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At least 32 people were transferred across the border into Turkey for treatment, and Ankara said autopsies performed on three people who died in Turkish hospitals confirmed chemical weapons had been used.

"Autopsies were carried out on three of the bodies after they were brought from Idlib. The results of the autopsy confirms that chemical weapons were used," Bekir Bozdag, Turkey's justice minister, said on Thursday, according to state-run Anadolu news agency.

"This scientific investigation also confirms that Assad used chemical weapons," Bozdag added, without giving further details.

The World Health Organization and Doctors Without Borders (MSF) said victims showed symptoms consistent with the possible use of a nerve agent, such as sarin.

If confirmed, the suspected chemical attack would the deadliest since sarin gas killed hundreds of civilians in Ghouta near the capital, Damascus, in August 2013.

Western states said the Syrian government was responsible for the 2013 attack. Damascus blamed rebels.