Opposition activists claim that dozens of people have been killed in a suspected chemical attack in a town in Syria's northern Idlib province.
The Britain-based Syrian Observatory for Human Rights monitoring group put the death toll at 58, saying there were 11 children among the dead.
Meanwhile, the Idlib Media Centre said dozens of people had been killed.
Hours after the attack, a small field hospital in the region was struck and destroyed, according to a civil defence worker in the area.
It was the third claim of a chemical attack in just over a week in Syria. The previous two were reported in Hama province, in an area not far from Khan Sheikhoun, the site of Tuesday's alleged attack.
The Syrian American Medical Society, which supports hospitals in opposition-held territory, said it had sent a team of inspectors to Khan Sheikhoun and an investigation was under way.
The Syrian activists had no information on what agent could have been used in the assault. They claimed the attack was caused by an air strike carried out either by the Syrian government or Russian warplanes.
Russia's defence ministry categorically rejected the claim that its planes attacked the town with chemical weapons.
The ministry said that "Russian air force planes haven't dealt any strikes on Khan Sheikhoun in the province of Idlib".
A Turkey-based Syrian woman whose niece, husband and one-year-old daughter were among those killed said the warplanes struck early, as residents were still in their beds. Makeshift hospitals soon crowded with people suffocating.
The province of Idlib is almost entirely controlled by the Syrian opposition. It is home to some 900,000 displaced Syrians, according to the United Nations. Rebels and opposition officials have expressed concerns that the government is planning to mount a concentrated attack on the crowded province.
The reports came on the eve of a major international meeting in Brussels on the future of Syria and the region hosted by the EU's High Representative, Federica Mogherini.
Claims of chemical weapons attacks, particularly the use of the chlorine agent, are not uncommon in Syria's conflict. The worst attack was what a UN report said was an attack by toxic sarin gas in August 2013 on the Damascus suburb of Ghouta that killed hundreds of civilians.
In the wake of the 2013 attack, President Bashar Assad agreed to a Russia-sponsored deal to destroy his chemical arsenal and joined the Chemical Weapons Convention. His government declared a 1,300-ton stockpile of chemical weapons and so-called precursor chemicals that can be used to make weapons amid international outrage at a nerve gas attack on the outskirts of Damascus.
Those weapons have been destroyed, but member states of the OPCW have repeatedly questioned whether Assad declared everything in 2013. The widely available chemical chlorine was not covered in the 2013 declaration and activists say they have documented dozens of cases of chlorine gas attacks since then.
The Syrian government has consistently denied using chemical weapons and chlorine gas, accusing the rebels of deploying it in the war instead.
Tarik Jasarevic, spokesman for the World Health Organisation in Geneva, said in a statement that the agency is contacting health providers from Idlib to get more information about Tuesday's incident.
The Syrian Coalition, an opposition group based outside the country, said government planes carried out the air strike on Khan Sheikhoun, south of the city of Idlib, the provincial capital.
It said the planes fired missiles carrying poisonous gases, killing dozens of people, many of them women and children. The coalition described the attack as a "horrifying massacre".
Photos and video emerging from Khan Sheikhoun showed limp bodies of children and adults. Some are seen struggling to breathe while others appeared to be foaming at the mouth.
A doctor said his hospital in Idlib province received three victims, all with narrow, pinpoint pupils that did not respond to light.
Pinpoint pupils, breathing difficulties and foaming at the mouth are symptoms commonly associated with toxic gas exposure.
The opposition's Civil Defence search-and-rescue group, which released photos showing paramedics washing down victims, has not published a casualty toll.
The activist-run Assi Press published video of paramedics carrying victims from the scene on a pick-up truck.
The New York-based Human Rights Watch has accused the Syrian government of conducting at least eight chemical attacks using chlorine gas on opposition-controlled residential areas during the final months in the battle for Aleppo last year that killed at least nine civilians and injured 200.
Also, a joint investigation by the United Nations and the international chemical weapons watchdog determined the Syrian government was behind at least three attacks in 2014 and 2015 involving chlorine gas and the Islamic State group was responsible for at least one involving mustard gas.
The European Union's top diplomat said Assad's government must assume its responsibilities following reports of the attack in northern Syria.
EU foreign policy chief Federica Mogherini said that "the news is awful" and that Assad's government "has the primary responsibility of protecting its people and not attacking its people".
She said the attack "is a dramatic reminder of the fact that the first priority is, as in any conflict, stopping the fighting".
Meanwhile, Britain is urging Russia and China not to block action against those responsible for the suspected chemical attack, which it is calling "a war crime".
Britain and France have called for an emergency meeting of the UN Security Council, which is scheduled to hold its monthly meeting on Syria's chemical weapons on Wednesday. Diplomats said that meeting could be moved up to later on Tuesday.
Britain's UN Ambassador, Matthew Rycroft, said the attack "has all the hallmarks of a regime attack" because the Syrian government is the only party to the conflict equipped to deliver deadly chemicals.
Mr Rycroft called the attack "clearly a war crime" and indirectly criticised Russia and China for protecting Syria by vetoing previous council resolutions.
The ambassador said he hoped for a different approach from "the Security Council members who have previously used their vetoes to defend the indefensible".
Mr Rycroft said an emergency council meeting would "shine a spotlight on the heinous use of chemical weapons yet again" in Syria, rally support for action in the council, and put pressure on Russia and China "to hold to account those who used chemical weapons".
Later, Syria's Foreign Ministry said Damascus is committed to its obligations under the Chemical Weapons Convention it joined in 2013, and denied its military has used such ordinance in an attack against civilians in northern Syria.
In comments to the official news agency SANA, a Foreign Ministry official said the country's military has no chemical weapons of any type and has "not used them before or later and doesn't seek to acquire them".
Syria agreed to destroy its chemical arsenal in 2013 and joined the convention following an attack in a rebel-held suburb of Damascus that left hundreds dead.
The Foreign Ministry official blamed opposition fighters for carrying out the attack in the northern town Khan Sheikhoun.
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