BEIRUT, Lebanon — The United States blamed the Syrian government and its patrons, Russia and Iran, on Tuesday for one of the deadliest chemical weapons attacks in years in Syria, one that killed dozens of people in Idlib Province, including children, and sickened scores more.
A senior State Department official said the attack appeared to be a war crime and called on Russia and Iran to restrain the government of President Bashar al-Assad of Syria from carrying out further chemical strikes.
Britain, France and Turkey joined Washington in condemning the attack, which they also attributed to Mr. Assad’s government. The United Nations Security Council was scheduled to be briefed on the attack on Wednesday.
One of the worst atrocities attributed to the Syrian government since President Trump took office, it poses a potential policy dilemma for the administration, which would like to shift the focus in Syria entirely to fighting the Islamic State.
Just days ago, Secretary of State Rex W. Tillerson said that Mr. Assad’s fate “will be decided by the Syrian people,” and Nikki R. Haley, the United States ambassador to the United Nations, said that “our priority is no longer to sit there and focus on getting Assad out.”
On Tuesday, the White House called the attack a “reprehensible” act against innocent people “that cannot be ignored by the civilized world.”
But Sean Spicer, the White House spokesman, said the slaughter was unlikely to change the United States’ posture toward Mr. Assad because of the “political realities” in Syria.
“There is not a fundamental option of regime change as there has been in the past,” Mr. Spicer told reporters. “Somebody would be rather silly not acknowledging the political realities that exist in Syria. What we need to do is to fundamentally do what we can to empower the people of Syria to find a different way.”
He added that “these heinous actions by the Bashar al-Assad regime are a consequence of the last administration’s weakness and irresolution.”
“President Obama said in 2012 that he would establish a ‘a red line’ against the use of chemical weapons and then did nothing,” Mr. Spicer said.
Russia has insisted that it had no military role in the strike. But the State Department official, who briefed reporters on Tuesday, said that Russian officials were trying to evade their responsibility because Russia and Iran were guarantors of the Assad government’s commitment to adhere to a cease-fire in the peace talks the Kremlin helped organized in Astana, Kazakhstan, this year.
The official said that it appeared Russia was unable or unwilling to hold the Syrian government to the agreed cease-fire.
He reiterated that the attack on civilians appeared to be a war crime. The official, who could not be identified under the State Department’s protocol for briefing reporters, also asserted that even before the alleged chemical strike, the Trump administration had shelved the idea of cooperating militarily with the Assad government against the Islamic State.
Witnesses to the attack said it began just after sunrise. Numerous photographs and graphic videos posted online by activists and residents showed children and older adults gasping and struggling to breathe, or lying motionless in the mud as rescue workers ripped off victims’ clothes and hosed them down. The bodies of least 10 children lay lined up on the ground or under a quilt.
A few hours later, according to several witnesses, another airstrike hit one of the clinics treating victims, who had been farmed out to smaller hospitals and maternity wards because the area’s largest hospital had been severely damaged by an airstrike two days earlier.
Rescue workers from the White Helmets civil defense organization said that many children were among at least 50 dead and 250 wounded. Radi Saad, who writes incident reports for the group, said that volunteers had reached the site not knowing a chemical was present, and that five of them had suffered from exposure to the substance.
While chlorine gas attacks have become almost routine in northern Syria, this one was different, medical workers and witnesses said. Chlorine attacks usually kill just a few people, often those trapped in an enclosed space, and the gas dissipates quickly.
This time, people collapsed outdoors, and in much larger numbers. The symptoms were also different: They included the pinpoint pupils of victims that characterize nerve agents and other banned toxins. One doctor posted a video of a patient’s eye, showing the pupil reduced to a dot. Several people were sickened simply by coming into contact with the victims.
The opposition minister of health, Mohamad Firas al-Jundi, said in a video that he had been in a field hospital at 7:30 a.m. when more than 100 people arrived wounded or sickened. Many others, he said, were scattered to other clinics.
“The patients are in the corridors and on the floors of the operation rooms, the E.R.s and in the patient rooms,” he said. “I saw more than 10 deaths due to this attack.”
Symptoms, he said, included suffocation; fluid in the lungs with foam coming from the mouth; unconsciousness; spasms; and paralysis.
“It’s a shocking act,” he said. “The world knows and is aware of what’s happening in Syria, and we are ready to submit evidence to criminal laboratories to prove the use of these gases.”
Mariam Abu Khalil’s exam on the Quran was scheduled for sunrise, since the examiners reckoned that was the time when bombs were least likely to fall. That proved wrong.
Mariam, 14, a resident of Khan Sheikhoun, where the attack took place, had not yet reached the exam hall when she saw an aircraft drop a bomb on a one-story building a few dozen yards away. In a telephone interview Tuesday night, she described an explosion like a yellow mushroom cloud that stung her eyes. “It was like a winter fog,” she said.
Sheltering in her home nearby, she saw several residents arrive by car to help the wounded. “When they got out, they inhaled the gas and died,” Mariam said.
The attack appeared to be the largest and deadliest chemical attack in Syria since August 2013, when more than 1,000 people were killed in the Damascus suburbs by the banned toxin sarin. Under threat of United States retaliation, Mr. Assad agreed to a Russian-American deal to eliminate his country’s chemical weapons program, which until that time it had denied having, and to join an international treaty banning chemical weapons.
But the operation took far longer than expected and raised questions about whether all the materials were accounted for. The head of the international monitoring body, the Organization for the Prohibition of Chemical Weapons, complained in an internal report about misleading statements from Damascus and expressed concern about possible undeclared chemical weapons activities.
Since then, the organization has found that the Syrian government used chlorine gas as a weapon three times in 2014 and 2015, violating the treaty. Rebel fighters, doctors and antigovernment activists say there have been numerous other chlorine attacks, including at least two in the past week, in one case killing a doctor as he worked.
The government denies that it has used chemical weapons, arguing that insurgents and Islamic State fighters use toxins to frame the government or that the attacks are staged.
Leith Abou Fadel, the editor of a pro-government news site, citing military sources, wrote that the Syrian military had bombed a weapons factory belonging to insurgents, causing the release of the chemicals.
The Organization for the Prohibition of Chemical Weapons has also accused the Islamic State of using banned mustard gas in Iraq and Syria. But the area around Khan Sheikhoun is not held by the Islamic State but by other insurgents — Qaeda-linked militants and a variety of other rebel groups.
A chemical weapons attack, if carried out by the government, would be a brazen statement of impunity, coming during a major international meeting in Brussels where officials are debating whether the European Union and others will contribute billions of dollars for reconstructing Syria if it is presided over by a government run by Mr. Assad.
“Today’s chemical attack was a direct insult to the #EU,” Fadi Halisso, a Syrian former priest who runs Basmeh and Zeitooneh, a humanitarian organization that aids Syrian and Palestinian refugees, said on Twitter.
“Assad is telling them, ‘You will pay, and I will continue killing,’” he added from Brussels, where he was attending the meeting. “You can do nothing.”
There had already been debate about whether the European Union and other Western countries would be willing or able to insist on a significant political transition, or at least power sharing, as a condition for supplying reconstruction funds.
Khan Sheikhoun is on a supply route about 20 miles from the front lines of the battle in neighboring Hama Province between government forces and a mix of insurgents, including United States-backed groups and Qaeda-linked fighters.