(CNN) - US Ambassador to the UN Nikki Haley stated Friday that chemical weapons were used in an attack in Syria that left scores dead and she blamed Russia for complicity, as the Trump administration continues to weigh how to retaliate and Russia maintains its denial that the attack took place.
"Did a chemical weapons attack happen? Yes," Haley said, speaking to reporters before a meeting of the Security Council. "The US has analyzed, yes it has happened. The UK has analyzed, yes, it has happened. And France has analyzed, yes, it has happened. Three countries."
Later, speaking at the meeting, called by Russia to discuss the prospect of an American strike on Syria, Haley said, "This meeting should not be about so-called 'unilateral threats,' it should be about the multiple actions Russia has taken to bring us to this point."
She told the council that President Donald Trump "has not yet made a decision about possible action in Syria, but should the United States and our allies decide to act in Syria, it will be in defense of a principle on which we all agree."
"It will be in defense of a bedrock international norm that benefits all nations," Haley said.
Haley, who said she would be heading to Washington for meetings later Friday, added that the National Security Council has been cautious and methodical about a decision to retaliate over the attack on the last rebel-held stronghold in Syria, which left at least 49 dead.
"You don't rush decisions like this," she told reporters. "We're making sure we have all the information to know if we do something, what will happen, how will it happen and will it hurt anyone."
Top military officials, including Defense Secretary James Mattis, warned Trump during an afternoon meeting Thursday that he risks escalating US involvement in Syria if he goes forward with the type of aggressive bombing campaign he has pushed over the past week, according to US and Western officials briefed on the conversation.
Trump has pressed military leaders to develop plans for a sustained assault on Syrian regime targets in response to last weekend's chemical attack, the officials said. But Mattis and other members of his national security team cautioned Trump during the meeting that such a strategy could pull the US into direct conflict with Russia and Iran.
At the UN meeting, Syrian Ambassador Bashar Jaafari warned that Syria would respond if the US moved against his country.
"Those three countries -- the US, Britain and France -- if they think they can attack us and attack our sovereignty, at the time, we would have no other choice except applying Article 51 of the charter that gives us the right to defend ourselves, the legitimate right to defend ourselves," Jaafari said. "This is not a threat. This is a promise. This is a promise. We will not let anyone attack our sovereignty."
Russia continues to dispute that the attack took place at all.
Haley spoke directly after Russian Ambassador to the UN Vassily Nebenzia, who charged that the Saturday attack on Douma, the last rebel-held town in Syria, was faked by foreign intelligence services.
"There is no credible proof of this," Nebenzia said. "We have weighty justification to believe ... that what took place was a provocation of certain countries' intelligence services."
Nebenzia was echoing Russian Foreign Minister Sergey Lavrov, who said in a Friday news conference that Russia has "irrefutable evidence" the alleged chemical attack in Syria was "staged."
He did not say what the evidence was. He was speaking a day after French President Emmanuel Macron said France had proof there was a chemical attack -- also without revealing what that evidence was.
Speaking in Moscow, Lavrov said a mission from the Organization for the Prohibition of Chemical Weapons was due to arrive in Damascus on Saturday and would plan to go to Douma "without delay," where Russian specialists had found "no evidence" of the use of chemical weapons or chlorine.
"We have irrefutable evidence that this was another staged incident and the special services of one state at the forefront of the Russophobic campaign were at play," Lavrov claimed.
Nebenzia told the Security Council "before the investigation even occurred, guilty parties were identified." The US and its allies, Nebenzia alleged, are setting the stage to "unleash a military scenario against Syria." This scenario, he said, "cannot be tolerated ... especially in light of the deployment in Syria of the Russian contingent."
As Haley began her remarks, she broke from the prepared text to say, "I started to listen to my Russian friend and respond, but instead I am in awe, Vassily, of how you say what you say with a straight face. I really, really am."
Pentagon spokesman Maj. Adrian Rankine-Galloway said the "Russian accusations seeking to shift blame away from the Assad regime for the April 7 chemical attack have no basis in fact and are typical of Russian disinformation efforts.
Biological samples from the area of the attack in Syria have tested positive for chlorine and a sarin-like nerve agent, according to a US official who's familiar with the US analysis of the test results. A Western official told CNN that it is not conclusive, but officials suspect the substance used in the attack was a mixture of chlorine, sarin and possibly other chemicals.
Another US official familiar with how the US intelligence is unfolding regarding the attack tells CNN that the initial intelligence assessment that a chemical agent was used was based on viewing the videos and comparing the shown physical effects with what is known about chlorine and nerve agents.
UN Secretary-General Antonio Guterres opened the Security Council meeting without referring to either Russia or the US, but stressed his deep concern that the growing tensions could spiral into broad conflict.
"Syria indeed today represents the most serious threat to international peace and security," Guterres said. "Increasing tensions and the inability to reach a compromise in the establishment of an accountability mechanism threaten to lead to a full-blown military escalation."
Guterres also said a failure to hold someone accountable for the attack could encourage more. "A lack of accountability emboldens those who would use such weapons by providing them with the reassurance of impunity," the secretary-general said.
Haley kept up the pressure on Russia. She noted in the meeting that Russia "has stopped at nothing to defend the Syrian regime's multiple uses of chemical weapons."
She noted that In 2017, Moscow "killed" a Joint Investigative Mechanism, which the UN established in 2015 to investigate and assign accountability for chemical weapons use in Syria. Russia has also used its UN veto six times to prevent condemnation of Assad's use of chemical weapons.
"To make matters worse, it is Russia alone that had agreed to be the guarantor of the removal of all chemical weapons from Syria," Haley said. "If Russia had lived up to its commitment, there would be no chemical weapons in Syria, and we would not be here today."
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