Vassily Nebenzia, Moscow's ambassador to the United Nations, said he "cannot exclude" war between the United States and Russian Federation and urged Washington and its allies to refrain from military action against Syria.
Russian Federation continues to dispute that the attack took place at all.
Mattis' remarks at a House Armed Services Committee hearing followed a series of Trump tweets this week that initially indicated he was committed to bombing Syria but later suggested he was awaiting further advice and assessment.
Russia's Defence Ministry said on Friday that Britain was involved faking a suspected chemical weapons attack in the Syrian town of Douma.
The US gave no details of civilian or military casualties, but its military leaders insisted targets had been chosen carefully to minimise civilian casualties, but maximise the message intended for the Syrian government of President Bashar al-Assad.
French President Emmanuel Macron on Thursday said France has proof that the Syrian government launched chlorine gas attacks and that France would not tolerate "regimes that think everything is permitted".
The OPCW said it would "shortly" deploy a fact-finding team to Douma for an investigation, but United States officials said they were working from their own information and would not necessarily hold back.
Mattis said he personally believes Syria is guilty of an "inexcusable" use of chemical weapons, while noting that the worldwide fact-finding team would likely fall short of determining who was responsible.
"But also it's a very, very delicate circumstance and we've got to make this judgment on a very careful, very deliberate, very well thought-through basis".
Moscow, he said, has "irrefutable information that it was another fabrication".
Lavrov added that the Swiss lab also pointed at the presence of the nerve agent A234 in the samples, but added that the lab noted that its presence in the samples appeared unusual, given the substance's high volatility and the relatively long period between the poisoning and the sample-taking.
He said he would also strive to prevent an escalation of conflict across the Middle East.
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The more capable defenses are arrayed to protect Russian military assets, centered at the Hmeimim air base in northwest Syria, rather than Syrian military facilities across the country.
Russian Federation entered Syria's civil war in 2015 and has backed the Assad regime.
"They are the guarantors of law and order in the town", RIA news agency quoted Russia's defence ministry as saying. "We are continuing to assess intelligence and are engaged in conversations with our partners and allies".
He noted that the Russian and US militaries have a hotline to prevent incidents, adding that it's not clear if it would be sufficient amid mounting tensions.
World stocks edged down as anxious investors stayed wary of risky assets. But, she said, "We can not allow such grievous violations of worldwide law". A U.S. guided-missile destroyer, the USS Donald Cook, is in the Mediterranean.
"To make matters worse, it is Russian Federation alone that had agreed to be the guarantor of the removal of all chemical weapons from Syria", Haley said.
Mattis said the president's National Security Council would be meeting later Thursday to "take forward the various options" for Trump.
The White House said such targeted sanctions would help to ensure that Russian oligarchs profiting from the Kremlin's destabilising activities, including its interference of Western democratic elections in 2016 and 2017, would face consequences for their actions.
Inside the briefing room laden with chandeliers, parquetry floors and gilded oil paintings, Ambassador Yakovenko outlined his concern about the dual crisis that have plunged Western relations with Russian Federation to lows not seen since the Cold War this week to the world's press. "If we want to do that, we the Western partners can not diverge in our approaches".
Trump had repeatedly said he would not telegraph military moves against foes such as North Korea and Islamic State.
She would not seek approval from Parliament, the BBC said, despite calls from the opposition Labour Party for Parliament to be given a say.