President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE on Monday promised a swift response to an apparent chemical weapons attack in Syria, insisting that those responsible will “pay a price.”

“We'll be making that decision very quickly, probably by the end of today,” Trump said during a Cabinet meeting, adding, “We cannot allow atrocities like that. Cannot allow it.”

The president condemned the “heinous attack,” which left at least 40 people dead, including children, and has been blamed on Syrian President Bashar Assad. Trump also suggested that Russian President Vladimir Putin, one of Assad’s closest allies, shoulders some of the blame.

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“He may. Yeah, he may,” Trump said when asked about Putin, whom he has been reluctant to criticize by name. “And if he does, it's going to be very tough. Very tough. Everybody is going to pay a price. He will. Everybody will.”

The alleged strike on civilians in the former opposition stronghold of Douma drew international condemnation and sent the White House scrambling to respond.

John Bolton spent his first morning on the job as Trump’s national security adviser leading a meeting with senior officials to discuss possible responses. In 2013, Bolton opposed a possible military strike on Syria in response to a sarin gas attack carried out by Assad. Then-President Obama decided against a military response.

Trump was also set to discuss options later Monday with his national security team and military brass.

“Nothing is off the table,” Trump said.

The president said the U.S. is still working to determine who was behind the suspected attack.

“If it's Russia, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out and we'll know the answers quite soon,” he said. “So we're looking at that very, very strongly and very seriously.”

The State Department told Reuters on Monday that victims’ symptoms were consistent with an asphyxiation or nerve agent. Russian Foreign Minister Sergey Lavrov said there was no evidence that a chemical weapons attack took place in Douma, in Damascus's northeast suburbs.

Moscow’s denials left the U.S. and its allies unconvinced.

Speaking at an emergency meeting of the United Nations Security Council, U.S. Ambassador to the U.N. Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE pointed a finger at Russia.

“The Russian regime, whose hands are all covered in the blood of Syrian children, cannot be ashamed by pictures of its victims,” Haley said. “We’ve tried that before. We must not overlook Russia and Iran’s roles in enabling the Assad regime’s murderous destruction.”

Haley also pledged the U.S. would respond “either way,” regardless of whether the U.N. does.

Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE suggested Russia did not uphold a deal to help dispose of Assad’s chemical weapons arsenal.

“The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons,” Mattis told reporters at the Pentagon.

Mattis, who is one of Trump’s most trusted advisers, also did not rule out a military strike.

The Syrian and Russian governments blamed Israel for an airstrike on a major Syrian military base overnight that reportedly killed 14 people. Israel has not claimed responsibility for the attack.

While the chemical attack was still being investigated, Republicans on Capitol Hill urged Trump to act without delay.

Last April, Trump launched a barrage of cruise missiles against a Syrian air base in response to a sarin gas attack that killed more than 80 people — the type of response many lawmakers said the president should carry out again.

“The president responded with a targeted strike against those Syrian military units that carried out the attack,” Rep. Mac Thornberry William (Mac) McClellan ThornberryTrump payroll-tax deferral for federal workers sparks backlash Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq Top Armed Services Republican 'dismayed' at Trump comments on military leaders MORE (R-Texas), chairman of the House Armed Services Committee, said on Fox News. “I thought that made very good sense, and it seems to me we ought to consider doing it again with our allies.”

Sen. Lindsey Graham Lindsey Olin GrahamGraham: GOP will confirm Trump's Supreme Court nominee before the election Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee MORE (R-S.C.), a longtime advocate for action against Assad, argued Monday for more aggressive action than the limited strike conducted last year. Specifically, Graham wants to the U.S. to destroy Assad’s air force or even target Assad himself.

“I think now he is a legitimate war criminal in the eyes of the international community,” he said on “Fox & Friends." “If you have the opportunity to take him out, you should.”

Sarin is banned under an international chemical weapons treaty. It remains unclear what type of substance was used in last weekend’s attack, which could complicate discussions about a response.

James Carafano, a defense policy expert at the Heritage Foundation who is close with the Trump administration, said the U.S. should respond similarly to the latest reports of a chemical weapons attack.

He argued that something more, such as destroying Assad’s air force, risks plunging the U.S. into the Syrian civil war and possibly an armed conflict with Russia and Iran.

“Our interest is not to solve Syria. Our interests are to make sure [the Islamic State in Iraq and Syria] doesn’t have a sanctuary, that the conflict doesn’t bleed into Iraq and Jordan, and that we do not have a repeat of masses of migrants,” Carafano said.

Some lawmakers, however, said Trump should seek congressional approval before launching a strike.

Sen. Mike Lee Michael (Mike) Shumway LeeMcConnell shores up GOP support for coronavirus package McConnell tries to unify GOP Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (R-Utah), a frequent critic of military intervention, said a strike is warranted but that Trump “should come to Congress and ask for authorization before military force is used."

Critics said Trump’s strike last April was ultimately ineffective because there was no meaningful follow-up to ratchet up pressure on Assad.

The reports of the chemical weapons attack came just days after Trump announced he wants to withdraw U.S. forces from Syria, comments that GOP critics said emboldened Assad to use increasingly brutal tactics against his own people.

Senate Armed Services Committee Chairman John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.) said Sunday that Assad, Russia and Iran “heard” Trump’s desire to withdraw from Syria, “and emboldened by American inaction, Assad has reportedly launched another chemical attack against innocent men, women and children.”

Responding to McCain, White House press secretary Sarah Huckabee Sanders called it “outrageous to say the president of the United States green-lit something as atrocious as the actions that have taken place over the last several days.”

Robert Ford, the United States' former ambassador to Syria, said he does not think Trump’s talk of withdrawing led Assad to conduct the attack, pointing to the leader’s regular use of chemical weapons since 2013.

Ford said the U.S. cannot change the outcome of the war. But he argued that the U.S still needs to deter the use of chemical weapons in Syria and can only do so by being consistent in its response.

“In order to deter future use, Assad needs to understand that every single time he use chemical weapons, America will strike. Establishing deterrence that way cannot be done by one big shock-and-awe strike,” said Ford.