On Saturday, dozens of people were killed in Syria in a suspected chemical attack. Aid groups are blaming President Bashar al-Assad’s government for the assault; United States President Donald Trump is threatening that the regime will face a “big price” for its actions if it attacked its own citizens.

The Syrian Civil Defense and the Syrian American Medical Society said in a joint statement on Sunday that hundreds of people had gone to medical centers in and around the city of Douma, East Ghouta, on Saturday “with symptoms indicative of exposure to a chemical agent.” Dozens of people died: Reports of the death toll ranged from the 40s to 150 or more.

Antigovernment activists circulated videos of lifeless men, women, and children purported to be victims of the chemical attack. “Douma city, April 7 … there is a strong smell here,” a voice can be heard saying on one of the videos, as reported by Reuters.

Syrian state news media denied that government forces had been involved and accused the Islamist rebel group that controls Douma, the Army of Islam, of fabricating the videos. Russia, an ally of the Assad regime, is also denying claims that chemical weapons had been used, and the foreign ministry of Iran, also an Assad ally, said reports of the gas attack were not based on facts but instead “an excuse” by the United States and Western countries to take military action.

In a statement, the US State Department said that the reports of the attack, if confirmed, “are horrifying and demand an immediate response by the international community.” The UK’s foreign secretary, Boris Johnson, said the reports were “deeply disturbing” and warned Russia not to try to block an international investigation.

The United Nations Security Council will meet on Monday regarding the attack. Nikki Haley, United States ambassador to the UN, said in a tweet of a statement on the meeting that “strong action is needed” on Syria.

We along with 8 other members have called for an emergency Security Council mtg tomorrow in reference to the horrible chemical weapons attack on innocent civilians in Syria. This is becoming all too common. Strong action is needed. pic.twitter.com/Vt3L5fjVla — Nikki Haley (@nikkihaley) April 8, 2018

Assad has won back control of almost all of the eastern Ghouta region in Syria since February. It has been one of the deadliest offensives in the country’s seven-year war, killing more than 1,600 civilians.

President Trump attacked “animal Assad” and threatened a “big price” on Twitter

On Sunday, President Trump reacted to the reports of the chemical attack in Syria in a series of tweets. He targeted the Syrian government and its allies, threatening a “big price” to pay over the assault, which he characterized as “SICK!”

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. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price... — Donald J. Trump (@realDonaldTrump) April 8, 2018

....to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK! — Donald J. Trump (@realDonaldTrump) April 8, 2018

In April 2017 Trump ordered military strikes on a Syrian air base after a chemical attack there killed more than 80 civilians. More recently, however, he has said he wants to withdraw US military troops from Syria; this month he instructed his military commanders to wrap up operations there and bring American troops home.

White House Homeland Security and Counterterrorism adviser Tom Bossert said in an interview on ABC’s This Week with Martha Raddatz that chemical attacks are an “unacceptable practice” and wouldn’t rule out any possible actions from the United States. “I wouldn’t take anything off of the table,” he said. “These are horrible photos, we’re looking into the attack at this point.”

In a separate interview on This Week, Sen. Lindsey Graham (R-SC) said Trump’s response to the attack in Syria would be a “defining moment” in his presidency. “They see us, our resolve, breaking. They see our determination to stay in Syria waning. And it’s no accident they used chemical weapons,” Graham said.

Trump on Sunday blamed Russian President Vladimir Putin, Russia, and Iran for the reported chemical attack in Syria — and also former US President Barack Obama. Trump said if not for Obama’s military inaction, “the Syrian disaster would have ended long ago.”

If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history! — Donald J. Trump (@realDonaldTrump) April 8, 2018

Obama declared the use of chemical weapons a “red line” in 2012, but after such an attack in 2013, he declined to respond using military force. As CNN’s Jake Tapper pointed out on Twitter, Trump in 2013 publicly urged Obama not to intervene in Syria.

Obama has called Syria “one of the hardest issues” of his presidency.

And regardless of Obama’s choices, what to do is now in Trump’s hands. Of Trump’s Twitter threat, Graham, a military hawk, said on Sunday that what matters is action behind the tweets. “If it becomes a tweet without meaning then he has hurt himself in North Korea. If he doesn’t follow through and live up to that tweet, he’s going to look weak in the eyes of Russia and Iran,” Graham said. “So this is a defining moment, Mr. President. You need to follow through with that tweet.”