At least 58 people, many of them children, were reportedly killed on Tuesday morning by a "toxic gas" attack in northwest Syria, a monitor said.

The London-based Syrian Observatory for Human Rights (SOHR) reported that an airstrike on the rebel-held town of Khan Sheikhun was responsible for the civilian deaths.

SOHR, which relies on witness reporting from within Syria and is sometimes accused of a favorable stance towards the opposition forces, said it was trying to determine what the substance was and if it had been dropped by Syrian aircraft or planes belonging to allied Russia. 

Russia and Syria denied carrying out a chemical weapons attack.

The Syrian government had promised to destroy its store of chemical weapons in 2013 after a deadly sarin attack on a Damascus suburb left hundreds, prompting a threat of US military action. In 2014, the Organization for the Prohibition of Chemical Weapons said it had removed Syria's chemical weapons stockpiles.

The Syrian government has since been accused of carrying out multiple chlorine gas attacks not covered by US-Russia brokered deal to remove the chemical weapons stockpiles.  Rebels and the "Islamic State" have also been accused of chemical weapons attacks during the six-year war. 

It was not clear what substance was used, but the opposition Syrian National Coalition accused the regime of using sarin.

Syria's army said terrorist groups used "chemical and toxic substances" in an attempt to frame the government. The Russian Defense Ministry said it assessed a Syrian airstrike hit a known arsenal and ammunition factory. It said the airstrike by Syrian jets caused toxic agents to be released.  

Photographs collected by activists showed some of the White Helmets volunteer rescue group hosing down victims with water, and two men foaming at the mouth after the attack. Medical workers in the town of Khan Sheikhun told SOHR that victims had been brought in vomiting and fainting after the air raid, and on top of the dead there were dozens of patients suffering respiratory problems. 

The Syrian Medical Relief Group, an international aid agency funding hospitals in Syria, said at least 400 people were injured in the attack.

UN rights investigators launch investigation

Later on Tuesday, United Nations human rights investigators began gathering information on the alleged chemical weapons attack after the National Coalition - an opposition group uniting more moderate elements - demanded an independent investigation.

"The National Coalition demands the Security Council convene an emergency session... open an immediate investigation and take the necessary measures to ensure the officials, perpetrators and supporters are held accountable," the group said in a statement.

UN investigators also said they were probing whether a medical facility treating victims had been hit in a separate airstrike. 

The UN Commission of Inquiry on Syria said that "both the use of chemical weapons, as well as the deliberate targeting of medical facilities, would amount to war crimes and serious violations of human rights law."

Idlib province, where Khan Sheikhun is located, is mostly controlled by the Tahrir al-Sham alliance, which is dominated by the Fateh al-Sham Front, formerly known as the al-Qaeda affiliated al-Nusra Front.

White House: 'Heinous' attack consequence of Obama administration's weakness

The White House condemned the Syrian government for carrying out what it described as a "heinous" attack and said that US President Donald Trump was "extremely alarmed" by this "intolerable act."

Spokesman Sean Spicer described the attack as "reprehensible" and that it could not be ignored by the civilized world.

Spicer said the Assad regime's actions were a consequence of the Obama administration's  "weakness and irresolution" in handling the Syrian crisis. Obama said he would draw a "red line" at chemical attacks, "then did nothing," the White House press secretary said.

The spokesman also refused to say whether the Trump administration believed that Russia had played a role in the attack.

President Donald Trump's suggestion last week that removing President Bashar al-Assad from power might no longer be the top US priority  would represent quite a shift in policy for Washington in the six-year civil war.

US Secretary of State Rex Tillerson also raised eyebrows last week when he said that al-Assad's fate would be "decided by the Syrian people."

After Trump last week walked back from the demand that Assad step down, EU foreign ministers said they saw no place for the strongman in Syria.

International outcry

Leaders across the world condemned the Syrian government and its allies following reports of the attack, and called for the  Syrian president to be held to account.

German Foreign Minister said on Twitter that if reports of the alleged chemical attack were confirmed, they would amount to "an act of almost unparalleled cruelty."

French President Francois Hollande accused the Syrian regime of carrying out a "massacre."

"Once again the Syrian regime will deny the evidence of its responsibility for this massacre," Hollande said. "Those who support this regime can once again reflect on the enormity of their political, strategic and moral responsibility."

British Foreign Secretary Boris Johnson said the attack "bears all the hallmarks" of the Syrian government. Johnson added that the UK government would "continue to lead international efforts to hold perpetrators to account."

The UN Security Council will gather on Wednesday after the US, UK and France called for an emergency meeting and an investigation into the attack. Britain's UN Ambassador Matthew Rycroft urged Russia and China not to veto any council resolution against those responsible.

EU foreign policy chief Federica Mogherini said that Tuesday's attack served as a "dramatic reminder of the fact that the first priority is, as in any conflict, stopping the fighting," adding that the Assad regime had the "primary responsibility of protecting its people and not attacking its people."

Turkey's foreign minister, Mevlut Cavusoglu, called the attack "a crime against humanity" but also criticized the West for failing to intervene for similar attacks in the past. Western nations, he said, were glad to give frequent lectures to the Middle East on human rights but "remained carefree when the red line was crossed before."

The attack came as the European Union was preparing to hold a two-day summit on the Syrian conflict in Brussels.

cw,es,dm/jm (AFP, Reuters)