Warning: This article contains distressing footage and images
Video report by ITV News Correspondent Geraint Vincent
Russia has vowed to continue its support of the Assad regime despite the deaths of at least 86 Syrian civilians in alleged chemical attacks.
The UK-based Syrian Observatory for Human Rights said those killed in Tuesday's attack on the town of Khan Sheikhoun included 30 children and 20 women.
Foreign Secretary Boris Johnson said all the evidence points to the Syrian regime under President Bashar al-Assad being responsible for the attack.
The UN held emergency talks on Wednesday to discuss the fatal contamination, which was condemned as "barbaric" by world leaders.
The US Ambassador to the UN called on Russia to help put an end to the "horrific acts" taking place in Syria, asking: "How many more children have to die for Russia to care?"
"If Russia has the influence in Syria that it claims to have, we need to see them use it," Nikki Haley told the meeting.
"We need to see them put an end to these horrific acts."
In response, Syria's US ambassador continued to deny that the Assad regime had used chemical weapons, while the Russian representative listened in silence.
A Kremlin spokesman said Russia will argue that the contamination of the northwestern province of Idlib was caused by exposure to chemicals from a rebel arsenal hit by a Syrian air strike.
Germany has argued that Russia should bear some responsibility for the deadly attacks, along with fellow Assad allies Iran.
A spokeswoman for the German government said that without "massive military support" from Russia and Iran, Syria would be forced to agree to serious talks on a political solution.
Boris Johnson said all the evidence he had seen suggested the attack was carried out by the Assad regime "in the full knowledge they were using illegal weapons in a barbaric attack on their own people".
Mr Johnson added a political process is needed "to get rid of that regime and to give the people of Syria a chance".
He said: "I would like to see those culpable pay a price for this."
Mr Johnson also called on "all members" of the UN Security Council to "condemn the chemical weapons attack."
The World Health Organisation said it is "alarmed" by the use of highly toxic chemicals as weapons.
"Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents," a WHO statement said.
Theresa May said she was "appalled" by the apparent chemical attack and called for an urgent investigation.
The Prime Minister, currently on a trade and security visit to the Middle East, said: "We condemn the use of chemical weapons in all circumstances. If proven [to be a chemical attack] it is further evidence of the barbarism of the Syrian regime.
"The UK has led international efforts to call to account the Syrian regime and Daesh and the use of chemical weapons.
"I am very clear there can be no future for Assad in a stable Syria, and I call on all parties involved to ensure we have a transition away from Assad."
US President Donald Trump also blamed the Assad regime for the attack and claimed it was consequence of the Obama administration's "weakness and irresolution".
In a press conference he said the attack had crossed "many lines".
"When you kill innocent children, innocent babies, with a chemical gas that is so lethal that people were shocked to hear what gas it was, that crosses many, many lines," he said.
Matthew Rycroft, the UK's ambassador to the UN, earlier told reporters in New York that the attack was "very bad news for peace in Syria".
He said: "This is clearly a war crime and I call on the Security Council members who have previously used their vetoes to defend the indefensible to change their course.
"We need to wait for the full investigation to take place, but, as I said, this bears all the hallmarks of a regime attack.
"It is only the regime that have the necessary equipment to deliver an attack like this and I've seen the same reports that you have about what the weapon might have been."