Donald Trump has called a suspected chemical attack in Syria the result of the "weakness" of the Obama administration.
The White House blamed the strike on Idlib province - which killed dozens and injured hundreds - directly on the government of President Bashar al-Assad.
In a statement Mr Trump said the incident was “reprehensible and cannot be ignored by the civilised world".
However, he also made clear that his administration believed the attack occurred thanks to the actions of the President's predecessor, Barack Obama
“These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution,” Mr Trump said.
“President Obama said in 2012 that he would establish a 'red line' against the use of chemical weapons and then did nothing.”
White House Press Secretary Sean Spicer earlier declined to say what the Trump administration would do about the attack but added that the President had spoken on Tuesday with his national security team about the issue.
The head of the health authority in rebel-held Idlib said more than 50 people had been killed and 300 wounded. The Union of Medical Care Organisations, a coalition of international aid agencies that funds hospitals in Syria, said at least 100 people had died.
Mounzer Khalil, head of Idlib's health authority, said hospitals in the province were overflowing with victims. “This morning, at 6.30am, warplanes targeted Khan Sheikhoun with gases, believed to be sarin and chlorine,” he told a news conference.
War planes later struck near a medical point where victims of the attack were receiving treatment, according to activists in the area.
The British-based Syrian Observatory for Human Rights said the attack killed at least 58 people and was believed to have been carried out by Syrian government jets. It caused many people to choke, and some to foam at the mouth.
Director Rami Abdulrahman told Reuters the assessment that Syrian government warplanes were to blame was based on several factors such as the type of aircraft, including Sukhoi 22 jets, that carried out the raid.
The Syrian army command has denied it had carried out the attack that killed up to 100 people, including children.
“We deny completely the use of any chemical or toxic material in Khan Sheikhoun town today and the army has not used nor will use in any place or time, neither in past or in future,” the army statement said.
The incident reported at Khan Sheikhoun would mark the deadliest chemical attack in Syria since sarin gas killed hundreds of civilians in Ghouta near Damascus in August 2013.
The Russian Defence Ministry said its aircraft had not carried out the attack. The UN Security Council is set to meet on Wednesday to discuss the incident.
Britain said he would be guilty of a war crime if it were proved his regime was responsible. The UN envoy for Syria said the “horrific” chemical attack had come from the air.
Photographs appeared to show people breathing through oxygen masks and wearing protection suits, while others carried the bodies of dead children, and corpses wrapped in blankets lined up on the ground.
Activists in northern Syria circulated pictures on social media showing a man with foam around his mouth, and rescue workers hosing down almost-naked children squirming on the floor.
A series of investigations by the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) found that various parties in the Syrian war had used chlorine, sulfur mustard gas and sarin.
A joint UN-OPCW report published in October said government forces used chlorine in a toxic gas attack in Qmenas in Idlib province in March 2015. An earlier report by the same team blamed Syrian government troops for chlorine attacks in Talmenes in March 2014 and Sarmin in March 2015. It also said Islamic State had used sulfur mustard gas.
The OPCW said it had begun “gathering and analysing information from all available sources” about the suspected Khan Sheikhoun attack.
France's Jean-Marc Ayrault called the attacks a "test" for the Trump administration and that President Trump and his team should clarify the US position towards Mr Assad.
“It's a test. That's why France repeats the messages, notably to the Americans to clarify their position,” Mr Ayrault told RTL radio, adding that it could do so when the UN Security Council meets.
“I told them that we need clarity. What's your position? The question is to know, yes or no, whether the Americans support a political transition in Syria, which means organising this transition, elections and that at the end of the process the question of Assad's departure is asked.”
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