US blames Assad as 58 dead in suspected chemical attack
11 children among those killed as Syria and Russia deny involvement
Dozens of people have been killed in a suspected chemical attack in a town in Syria's northern Idlib province.
Monitoring groups put the death toll at 58, saying there were 11 children among the dead. The Idlib Media Centre said dozens of people had been killed.
White House spokesman Sean Spicer condemned what he called "these heinous actions by the Bashar al-Assad regime", and said it was in the best interests of the Syrian people for President Assad to leave.
He was joined in his condemnation by the UK, United Nations and France, among others.
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 yesterday's 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 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".
The Syrian army command also denied it used any chemical weapons against civilians.
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 Assad agreed to a Russia-sponsored deal to destroy his chemical arsenal and joined the Chemical Weapons Convention.
His government declared a 1,300-tonne 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 Organisation for the Prohibition of Chemical Weapons have repeatedly questioned whether President 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, said in a statement that the agency is contacting health providers from Idlib to get more information about yesterday'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 emerged from Khan Sheikhoun showing limp bodies of children and adults.
Some were 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 United Nations Security Council will meet this morning to be briefed on yesterday's suspected chemical weapons attack