Warning: Some viewers may find the following footage disturbing.
- Video report by ITV News Correspondent Geraint Vincent
The UN Security Council will hold an emergency meeting on Wednesday to discuss reports of chemical attacks in Syria.
The UK and France requested the talks after reports of at least 100 people being killed and 400 injured by the attack in the rebel-held town of Idlib .
It is expected a draft resolution will be put forward calling for monthly reports on whether the Syrian government is cooperating with an international inquiry into chemical weapons.
Tuesday's attack saw fumes cause many people to choke, as pictures and video emerged showing people limping, struggling to breathe and even foaming at the mouth.
The Syrian Observatory for Human Rights (SOHR) said the incident was caused by a warplane releasing "toxic gas" in a residential area in the early hours of Tuesday.
They confirmed 11 children were among the dead and at least 160 were injured.
A short time later, a hospital in the town where doctors were treating the victims was bombed.
The entrance to the building was hit, bringing down rubble on top of medics who were earlier seen helping wash away chemical residue on patients.
US President Donald Trump blamed the Assad regime for the attack and claimed it was consequence of the Obama administration's "weakness and irresolution".
Russia denied involvement in the attack. Its Defense Ministry said intelligence suggested the attack was carried out by Syrian planes on weapons depots and an ammunition factory on the eastern outskirts of the town of Khan Sheikhoun.
- Video report by ITV News Washington Correspondent Robert Moore
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.
She added: "We cannot allow this suffering to continue".
The UN said reports of chemical weapons being used on civilians is "extremely alarming and disturbing".
Foreign Secretary Boris Johnson, who was meeting with his German counterpart Sigmar Gabriel for talks in London on Tuesday, described the attack as a "war crime" and said the Syrian government "must be held to account".
He told reporters: "If this were proved to have been committed by the Assad regime then it would be another reason to assume they are an absolutely heinous outfit.
"It is a war crime… bombing your own civilians with chemical weapons is unquestionably a war crime and they must be held to account.
"It is unbelievable to think that in the long-term Bashar al Assad can play a part in the future of Syria, given what he has done to his people.
"We have to find a way forward that leads to a transition away from Assad."
The attack is thought to be one of the worst since the war began six years ago.
The SOHR did not release any details on what agent could have been used in the attack, but said the strikes were carried out either by the Syrian government or Russian jets.
A military source told Reuters the Syrian army "does not and has not used chemical chemical weapons."
"Not in the past and not in the future," they added.
Russia has also denied any involvement saying they did not carry out any strikes near the area.
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 government in Damascus and Russia have denied responsibility.