The father of nine-month-old twins killed in the Syrian chemical attack has described cradling the infants' lifeless bodies in his arms before carrying them to a mass grave.
Abdel Hameed Alyousef's wife and twins were among more than 80 people killed in the attack on the Syrian town of Khan Sheikhoun on Tuesday.
Twenty-two of the victims were from Alyousef's immediate family and have been buried in a mass grave, according to Associated Press.
Footage from the aftermath of the attack shows Alyousef stroking his twins' hair and choking back tears before mumbling "Say goodbye, baby, say goodbye" to their lifeless bodies.
Alyousef sits in the front seat of a van with the twins, his eyes red as he asks his cousin Alaa to video his farewell to them.
When the attack took place, "I was right beside them and I carried them outside the house with their mother," Alyousef, a 29-year-old shopowner, told Associated Press.
"They were conscious at first, but 10 minutes later we could smell the odour."
The twins and his wife, Dalal Ahmed, quickly fell sick. Alyousef brought them to paramedics and, thinking they would be okay, went to look for the rest of his family.
He found the bodies of two of his brothers, two nephews and a niece, as well as neighbours and friends.
Only later was he told his children and wife had died.
"I couldn't save anyone, they're all dead now," Alyousef said.
Doctors say the victims of the attack show symptoms consistent with the use of a nerve agent such as sarin - suspected to have been used by Syrian government forces in deadly attacks outside Damascus in 2013.
US officials have not said what kind of agent they think was used, but US President Donald Trump said it was "a chemical gas that is so lethal, people were shocked to hear what gas it was".
Watch: A chemical gas attack kills scores
If confirmed, the attack would be among the worst incidents of chemical weapons use in Syria's civil war, which has killed over 320,000 people since it began in March 2011.
Syria officially relinquished its chemical arsenal and signed the Chemical Weapons Convention in 2013 to avert military action after it was accused of an attack outside Damascus that killed hundreds.
Watch: Suspected chemical gas attack came from the air
But there have been repeated allegations of chemical weapons use since.
The US ambassador to the United Nations, Nikki Haley, has warned of unilateral action and Secretary of State Rex Tillerson urged Russia to rethink its support for Assad.
"There is no doubt in our mind that the Syrian regime under the leadership of Bashar al-Assad is responsible for this horrific attack," Tillerson told reporters.
"And we think it is time for the Russians to really think carefully about their continuing support for the Assad regime."