US Ambassador Nikki Haley gave the warning during an emergency meeting of the UN Security Council to discuss the suspected gas attack in Syria.
The United States warned on Wednesday that it could take unilateral action if the UN fails to respond to a suspected chemical attack in Syria's Idlib province that killed dozens.
The warning came from the US ambassador to the UN, Nikki Haley, during an emergency meeting of the UN Security Council called by France and Britain.
"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," she said at the meeting without giving any signal what sort of action could be taken.
Haley 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 young child lying lifeless, a mask covering his face.
"How many more children have to die before Russia cares?" Haley 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," she added.
TRT World's Oliver Whitfield-Miocic reports.
The US, the UK and France presented a draft resolution, demanding a full investigation of the attack, however Russia has described it as "unacceptable."
The resolution calls on the Syrian regime to "cooperate fully" with a fact-finding mission by the Organization for the Prohibition of Chemical Weapons (OPCW).
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.
"What is your plan?"
Speaking in the council, British UN Ambassador Matthew Rycroft asked Russia "What is your plan? What is your plan to stop these horrific senseless attacks? We had a plan and we had the support and you rejected it to protect Assad."
Safronkov responded by saying Russia has more than one plan, but the first one is to fight terrorism.
Rycroft said those vetoes sent Assad a message of encouragement and Tuesday's attack was "the consequence."
An investigation by the UN and OPCW previously found that Syrian regime forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Daesh militants had used mustard gas.