U.S. Sen. Susan Collins said Sunday that the U.S. may need to consider a military strike against the Syrian government in response to a gas attack in a rebel-held town near Damascus that left at least 42 people dead, a move the White House is not ruling out.

“This attack with chemical weapons, which are banned by international conventions, is absolutely horrific,” Collins, a member of the Senate Intelligence Committee, said on CNN’s “State of the Union.”

The alleged gas attack hit the city of Douma late Saturday, Syrian opposition forces and rescue workers told the Associated Press, as Syrian government forces resumed an offensive after talks collapsed with the Army of Islam group.

The Syrian American Medical Society and Syrian Civil Defense said in a joint statement Sunday that more than 500 people, the majority of whom were women and children, were brought to local hospitals with symptoms ranging from difficulty breathing, foaming at the mouth, burning eyes and emitting a chlorine-like odor, symptoms the group allege indicate exposure to toxic chemicals.

The Syrian government strongly denied the allegations it had used chemical weapons against civilians, calling them “fabrications,” according to the Associated Press.

“The army, which is advancing rapidly and with determination, does not need to use any kind of chemical agents,” the Syrian government said in a statement, according to the Associated Press.

President Donald Trump on Sunday tweeted that there would be a “big price to pay” for the attack.

“Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world,” Trump tweeted, adding pointedly that Russian President Vladimir Putin and Iran also bear responsibility for the attack for “backing Animal Assad,” a reference to Syrian President Bashar al-Assad.

Vice President Mike Pence in a statement posted on Twitter before noon Sunday echoed Trump’s condemnation of the gas attack, saying “We condemn in the strongest possible terms the assault on innocent lives, including children. The Assad regime & its backers MUST END their barbaric behavior. As POTUS said, big price to pay for those responsible!”

On Sunday afternoon, the British mission to the United Nations said in a statement on Twitter that the U.N. Security Council is expected to meet Monday to discuss the reported gas attack after the United Kingdom, the United States, Poland, the Netherlands, Sweden, France and other nations called on the body to meet.

White House homeland security adviser Tom Bossert said Sunday on ABC’s “This Week” that no response to the attack is being taken “off the table,” saying “These are horrible photos, we’re looking into the attack at this point.”

It would not be the first time the Trump administration has used military force against the Syrian government for allegedly using chemical weapons on civilians. Last April, Trump ordered the U.S. military to launch 59 tomahawk missiles at a Syrian air base, where a deadly gas attack on the city of Idlib was believed to have originated, according to The Washington Post.

That’s an option Collins said the Trump administration should be considering.

“Last time this happened, the president did a targeted attack to take out some of the facilities, that may be an option we should consider now,” Collins said.

Collins last year commended Trump for the targeted attack on the Shayrat air base in Homs province, calling it a “decisive response to Assad’s appalling and indiscriminate attack against his own people.”

A spokeswoman for the senior senator also told the Bangor Daily News last year that Trump administration should consult Congress on use of military force against the Syrian government.

Collins in the past has been reluctant to support using military force against the Syrian government to curtail its use of chemical weapons, as such a move risks drawing the U.S. deeper into the now 7-year-old civil war.

Speaking to Jewish constituents at Temple Beth El in Portland in September 2013, Collins said “At this point, military strikes risk entangling the U.S. in a protracted and dangerous civil war,” adding that the U.S. has an “obligation to use nonmilitary options” to address the use of chemical weapons in Syria, according to the Portland Press Herald.

Her comments followed President Barack Obama’s decision to not use airstrikes against the Syrian government after it crossed a “ red line” with a chemical weapon attack that killed more than 1,400 Syrians near Damascus, a decision that Trump has criticized as recently as Sunday morning. Instead, Syria agreed to a Russian-brokered deal to give up its chemical weapons.

Saturday’s gas attack comes just days after Trump directed military leaders to prepare to withdraw the 2,000 U.S. troops currently in Syria, telling reporters on Tuesday that the mission there is “close to 100 percent” accomplished, and that “I want to get out. I want to bring our troops back home.”

Collins on Sunday appeared to cast doubt on whether now is the time to scale back the U.S. presence in Syria, telling CNN that “I think the president is going to have to reconsider his plan for an early withdrawal in light of what has happened.”

During her appearance on CNN, she also urged the Trump administration to ratchet up the pressure — particularly with sanctions — on Moscow for its support of Assad, saying “without the support of Russia, I do not believe that Assad would still be in office.”

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