President Bashar Assad's government came under mounting international pressure after a chemical attack in northern Syria, with even key ally Russia saying its support is not unconditional.

Turkey said samples from victims of Tuesday's attack on the northern opposition-held town of Khan Sheikhoun, which killed more than 80 people, indicate they were exposed to sarin, a highly toxic nerve agent.

Syria rejected the accusations, and Moscow warned against apportioning blame until an investigation has been carried out.

Meanwhile, discussions continued at UN headquarters on a Security Council resolution that would condemn the chemical attack.

Kremlin spokesman Dmitry Peskov said that "unconditional support is not possible in this current world".

But he added that "it is not correct to say that Moscow can convince Mr Assad to do whatever is wanted in Moscow. This is totally wrong".

Russia has provided military support for the Syrian government since September 2015, turning the balance of power in Assad's favour.

Moscow has used its veto power at the Security Council on several occasions since the civil war began six years ago to prevent sanctions against Damascus.

The two countries "enjoy a relationship of cooperation, of exchange of views and full mutual support," said Mr Peskov, a spokesman for President Vladimir Putin. Assad and his army are "the only real power in Syria that can resist terrorists on the ground," he said.

The Syrian government maintains it did not use chemical weapons, instead blaming opposition fighters for stockpiling the chemicals.

Russia's Defence Ministry said the toxic agents were released when a Syrian air strike hit a rebel chemical weapons arsenal and munitions factory on the eastern outskirts of Khan Sheikhoun.

"I stress, once again, that the Syrian Arab Army did not and will not use such weapons even against the terrorists who are targeting our people," Syria's foreign minister Walid Moallem told reporters in Damascus.

US President Donald Trump on Wednesday condemned the attack, saying it had crossed "many, many lines" and put the blame squarely on Assad's forces.

But Mr Trump refused to say what the US might do in response and was more reserved than many of his top advisers - including his UN envoy Nikki Haley, who strongly hinted some US action was coming.

German foreign minister Sigmar Gabriel welcomed Mr Trump's strong condemnation, but warned against a military escalation and urged the US to support UN-backed talks.

French counterpart Jean-Marc Ayrault urged a resumption of Syrian peace talks and said he wants Assad's government prosecuted over its alleged use of chemical weapons.

He told CNews television that a new UN resolution and Syrian peace negotiations should be a top priority - not rushing into new military interventions. Mr Ayrault said that "France is still seeking to talk with its partners on the Security Council... Russia in particular."

"These crimes must not remain unpunished... One day, international justice will rule on Assad," Mr Ayrault said.

The attack overwhelmed hospitals around Khan Sheikhoun, and paramedics sent victims to medical facilities across rebel-held areas in northern Syria, as well as to Turkey. The Britain-based Syrian Observatory for Human Rights put the death toll at 86.

The attack happened about 60 miles from the Turkish border, and the Turkish government - a close ally of Syria's rebels - set up a decontamination centre at a border crossing in Hatay province, where the victims were initially treated before being moved to hospitals.

Turkish officials said nearly 60 victims of the attack were brought to Turkey for treatment and three of them died.

Victims showed signs of nerve gas exposure, including suffocation, foaming at the mouth, convulsions, constricted pupils and involuntary defecation, the World Health Organisation (WHO) and Doctors Without Borders said. Paramedics used fire hoses to wash the chemicals from the bodies of victims.

The scenes were reminiscent of those that followed a 2013 nerve gas attack on the suburbs of Damascus, which left hundreds dead.

In Turkey, the state-run Anadolu and the private DHA news agencies quoted justice minister Bekir Bozdag as saying that "it was determined after the autopsy that a chemical weapon was used".

The Turkish Health Ministry later issued a statement saying that "according to the results of the first analysis, there were findings suggesting that the patients were exposed to chemical substance (sarin)".

Turkish media reported that WHO experts took part in the autopsies of Syrian victims at a hospital in the Turkish city of Adana late on Wednesday.

The Organisation for the Prohibition of Chemical Weapons said its Technical Secretariat has been collecting and analysing information about the allegations. "This is an ongoing investigation," it said.

At UN headquarters on Thursday, key Security Council members, including the US and Russia, were meeting behind closed doors to try to reach agreement on a resolution that would condemn the attack and authorise an independent investigation.

Britain's deputy ambassador Peter Wilson said: "What we want is a unanimous resolution... and we want to see this done soon."