She closed her remarks with an ominous warning. “When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action,” she said. “For the sake of the victims, I hope the rest of the council is finally willing to do the same.”
She did not provide further details. But she squarely blamed the government of President Bashar al-Assad for the attack, as did the Trump administration on Tuesday.
In Washington, Defense Secretary Jim Mattis, asked about the attack before his meeting with the Singaporean defense minister, said: “It was a heinous act and will be treated as such.”
The French ambassador to the United Nations, François Delattre, called on Russia to stand up to the use of chemical weapons and on the United States to show leadership on Syria. Mr. Delattre called the attack on Tuesday “a new act of barbarism.”
The British envoy, Matthew Rycroft, pushed his fellow diplomats to act or lose all credibility in the eyes of the public. “They view us as a table of diplomats doing nothing, our hands tied behind our backs, beholden to Russian intransigence,” he said.
Russia dismissed the comments, saying, “At this stage, we don’t see a particular need.” Its deputy ambassador, Vladimir Safronkov, went on to scold its drafters for expressing “horror” at the attack. “Have you even checked what you wrote? This draft was prepared in a hasty way.”
The Security Council meeting was adjourned and ambassadors from the five permanent members conferred privately. A vote on the draft resolution was not immediately scheduled.
Earlier on Wednesday, at a meeting in Brussels of donor countries for Syrian humanitarian relief convened by the European Union, António Guterres, the United Nations secretary general, declared that “war crimes are going on in Syria.”
Asked whether Mr. Assad’s government was responsible, Mr. Guterres called for “a very clear investigation to remove all doubts.”
Condemnation also came from Pope Francis, who called the attack “an unacceptable massacre”; the NATO secretary general, Jens Stoltenberg, who deplored “the use of these barbaric weapons”; and Donald Tusk, president of the European Council, who said that Syria’s government bore primary responsibility.
The Brussels conference is aimed at reassuring countries hosting millions of displaced Syrians — most notably Jordan, Lebanon and Turkey — that they will continue to receive financial support to manage the intense pressures the influx has caused.
The arrival of more than one million migrants, many of them Syrian, has bolstered the fortunes of right-wing populist groups in Germany, and the country’s foreign minister, Sigmar Gabriel, went out of his way to acknowledge the role Syria’s neighbors have played.
Lebanon and Jordan “have far fewer inhabitants than Germany, 10 percent, even less, and they’ve taken in an unbelievable number of refugees, and they’re relatively poor countries measured against European standards,” Mr. Gabriel said at a news conference, thanking them.
The war in Syria has taken nearly 400,000 lives, monitoring groups have said.
In the town of Khan Sheikhoun in Idlib Province, where the attack took place, rescue workers continued to look for and help survivors, some hiding in shelters.
The first known use of chemicals as weapons in Syria’s civil war came in 2012, and the attack on Tuesday was the most devastating since an August 2013 assault around the town of Ghouta that left hundreds dead and challenged President Barack Obama’s declaration that the use of chemical weapons would amount to a “red line.”
Mr. Obama considered a more direct American intervention in the conflict, but he ultimately decided against one.
“Doctors in Idlib are reporting that dozens of patients suffering from breathing difficulties and suffocation have been admitted to hospitals in the governorate for urgent medical attention, many of them women and children,” the World Health Organization said on Wednesday.
“Reports first emerged of the use of chemical weapons agents in Syria in 2012 and have since occurred with disturbing frequency,” the organization added, “including repeated allegations of chlorine use in and around Aleppo last year, especially from September to December 2016.”
Dr. Peter Salama, executive director of the organization’s Health Emergencies Program, said in a statement, “These types of weapons are banned by international law because they represent an intolerable barbarism.”
Hospitals in the Khan Sheikhoun area are stretched to the breaking point and Al Rahma Hospital — the first to treat victims of the attack — was itself temporarily rendered inoperable by bombing on Tuesday.
Another facility in the area, Ma’ara Hospital, “has been out of service since last Sunday because of extensive damage to infrastructure,” the World Health Organization reported. “Emergency rooms and intensive care units in Idlib are overwhelmed and reporting shortages in medicines required to treat injured patients. Many patients have been referred to hospitals in southern Turkey.”
The organization also said that some of the victims showed symptoms “consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents.”
Western governments, including the Trump administration, have blamed Mr. Assad for the attack, but the Syrian leadership and Russia, one of its principal backers, have denied responsibility.
In 2013, Mr. Trump urged Mr. Obama on Twitter, “Do NOT attack Syria, fix U.S.A.” But on Tuesday, even as the American president called the Syrian attack a “heinous” massacre that “cannot be ignored by the civilized world,” he blamed his predecessor, saying in a statement, “President Obama said in 2012 that he would establish a ‘red line’ against the use of chemical weapons and then did nothing.”
Even as the wrangling in Washington continued, the center of diplomatic efforts seemed to be at the United Nations, where Western officials feared that Russia would use the veto power it has as a permanent member of the Security Council to block condemnation of the latest attack.
The French foreign minister, Jean-Marc Ayrault, who met with his Russian counterpart, Sergey V. Lavrov, on Tuesday to express condolences for the St. Petersburg subway bombing this week, used the occasion to condemn the attack in Syria.
“He noted that this barbaric act should not go unpunished and recalled that the international community as a whole should take responsibility and work to establish facts and responsibilities,” the French Foreign Ministry said in a statement, referring to Mr. Ayrault.
Arriving at the Brussels conference — co-sponsored by the United Nations, Britain, Germany, Kuwait, Norway and Qatar — Boris Johnson, the British foreign secretary, said it was impossible to imagine Mr. Assad’s “barbaric regime” continuing after the conflict ends.
Mr. Johnson also suggested that Mr. Assad and his government should be held accountable for war crimes, regardless of whether Russia was involved in the latest chemical attack.
Speaking at the same news conference at which Mr. Gabriel appeared, Mr. Johnson said that money should not be used in any way that could help the government in Damascus, but he acknowledged that needed to be balanced against humanitarian needs.
“There can be no budget, there can be no European checkbook, no financing of Syria without a transition away from the Assad regime,” Mr. Johnson said.