WASHINGTON — Moving a step closer to possible US military action in Syria, a senior Obama administration official said Sunday that there was "very little doubt" that President Bashar Assad's military forces had used chemical weapons against civilians last week and that a Syrian promise to allow UN inspectors access to the site was "too late to be credible."
The official said that "based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts and other facts gathered by open sources, the US intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident."
The conclusion was described in a written statement released Sunday morning on the condition that the official not be named. It reflected a marked shift in tone after President Obama's meeting at the White House on Saturday with his national security team, during which advisers discussed options for military action.
The president, who warned a year ago that the use of chemical weapons by Syrian government forces would be a "red line," has faced criticism from congressional Republicans and others for failing to respond more forcefully to evidence of earlier, smaller-scale chemical attacks.
Obama, who inherited two costly wars — in Iraq and Afghanistan — has been extremely reluctant to commit US military forces, even in the form of missile strikes, to a tangled conflict in the Middle East.
But on Sunday, the White House seemed to take a harder line, dismissing the Syrian promise of possible access by UN inspectors. That raised at least the possibility that a strike on Syrian targets would come soon, perhaps using cruise missiles fired from ships off shore.
Obama spoke Sunday with the French president, Francois Hollande, and the White House said they had expressed "grave concern" about the reported Syrian chemical attack and "discussed possible responses by the international community."
Obama had spoken Saturday to the British prime minister, David Cameron. Analysts have said that Britain and France could well be part of an international coalition conducting strikes against Syria.
Early Sunday, the White House said Syrian officials had refused to let the inspectors see the site of the attack. But Syrian television subsequently reported that there was a deal to allow access beginning Monday.
The administration official who released the statement said the offer, even if sincere, might be meaningless because of the time that had already passed since the attack. "The evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days," the official said.
The official, however, did not suggest that Obama had decided to take action. "We are continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons," the official said.
But by labeling as "indiscriminate" the attack Wednesday in a Damascus suburb, which reportedly killed hundreds of civilians, the official suggested that the United States viewed the latest assault as different from the smaller suspected chemical attacks that had not brought US military action.
Several members of Congress from both parties said in televised interviews Sunday that they supported a proportional and quick US military response to the apparent use of toxic weapons.
The Syrian government has denied using chemical weapons, and Saturday it said its soldiers had found chemical supplies in areas seized from rebel forces. Russia, an ally of the Syrian government, accused the rebels of using the weapons, but few analysts think they have the supplies or ability to do so.
In a response that suggested concern about a possible imminent US attack, Moscow welcomed Syria's acceptance of inspections and cautioned against a rush to judgment.
A spokesman for the Russian Foreign Ministry, Alexander K. Lukashevich, said that those who advocated an armed response to any chemical weapons attack — without citing the United States or other countries — were prejudging the results of the UN inspections.
"In these conditions, we again resolutely call on all those who are trying to impose the results of the UN investigations and who say that armed actions against Syria is possible to show common sense and avoid tragic mistakes," Lukashevich said in a statement Sunday on the ministry's website.
Syria warned that any US military action would inflame the Mideast. Iranian state news media quoted the Tehran government as saying that any intervention by Washington would have hard consequences.
The attack Wednesday took place in an area behind rebel lines, which complicates the task of the UN team. While activists reached in the area said they would welcome the team, many are deeply skeptical that its work will result in action against Assad's government.
The team's original mandate was limited to determining whether or not chemical weapons had been used, not to assign responsibility for their deployment.
Israel sharpened its message Sunday, suggesting that the use of such weapons in the region should not go without a response. "This situation must not be allowed to continue," Prime Minister Benjamin Netanyahu said, referring to the Syrian civilians "who were so brutally attacked by weapons of mass destruction."
"The most dangerous regimes in the world must not be allowed to possess the most dangerous weapons in the world," Netanyahu said.
Some Israelis have argued that international intervention in Syria would distract the world from the crucial effort to prevent a nuclear Iran. But there is a growing sense among Israelis that Syria presents a test of how the world might respond to Iran as it approaches the capability of making a nuclear weapon.
"Assad's regime has become a full Iranian client, and Syria has become Iran's testing ground," Netanyahu added. "Now the whole world is watching. Iran is watching, and it wants to see what would be the reaction on the use of chemical weapons."
On Saturday, Doctors Without Borders, an international aid group, said that on the morning of the reported attack, medical centers it supported near the site received about 3,600 patients showing symptoms consistent with exposure to toxic nerve agents. Of those, 355 died, the group said.