The warning came from US Ambassador Nikki Haley during an emergency meeting of the UN Security Council called by France and Britain following the attack in the early hours Tuesday on a rebel-held town in Idlib province.
"When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action," Haley said, without elaborating.
The US ambassador lashed out at Russia for failing to rein in its ally Syria, standing in the council chamber to hold up photographs of victims -- one showing a child lying lifeless, a mask covering his face.
"How many more children have to die before Russia cares?" she asked.
"If Russia has the influence in Syria that it claims to have, we need to see them use it," she said. "We need to see them put an end to these horrific acts."
At least 72 people, among them 20 children, were killed in the strike on Khan Sheikhun, and dozens more were left gasping for air, convulsing, and foaming at the mouth, doctors said.
It is thought to be the worst chemical weapons attack in Syria since 2013, when sarin gas was used.
- France urges US to back Syria solution -
Britain, France and the United States have presented a draft resolution demanding a full investigation of the attack, but Russia said the text was "categorically unacceptable."
"If we are not prepared to act, then this council will keep meeting, month after month to express outrage at the continuing use of chemical weapons and it will not end," Haley said.
"We will see more conflict in Syria. We will see more pictures that we can never unsee."
The Western trio blame President Bashar al-Assad´s forces for the attack, but the Syrian army has denied any involvement.
French Ambassador Francois Delattre called on Russia to ratchet up pressure on Assad but also had a message for the Trump administration.
"Frankly we also need an American that is seriously committed to a solution in Syria and puts all its weight behind it. If not now, when?" Delattre told reporters.
The draft resolution backs a probe by the Organisation of the Prohibition of Chemical Weapons (OPCW) and demands that Syria cooperate to provide information on its military operations on the day of the assault.
Russia´s Deputy Ambassador Vladimir Safronkov told the council that the proposed resolution was hastily prepared and unnecessary, but voiced support for an inquiry.
"The main task now is to have an objective inquiry into what happened," he said.
Negotiations were continuing on the draft text, but diplomats said they hoped it could be put to a vote later on Wednesday.