WASHINGTON -- Moving a step closer to possible American military action in Syria, a senior Obama administration official said on Sunday that there was "very little doubt" that President Bashar al-Assad's military forces had used chemical weapons against civilians last week and that a Syrian promise to allow United Nations inspectors access to the site was "too late to be credible."
The official, in a written statement, 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 U.S. 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 statement, released on Sunday morning on the condition that the official not be named, reflected a tougher 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.
While administration officials emphasized that Mr. Obama had not decided to take action, they said he was determined not to be drawn into a protracted debate over gaining access for the United Nation investigators, given their doubts, at this point, that it would produce credible findings.
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. Mr. Obama, who inherited two costly wars -- in Iraq and Afghanistan -- has been extremely reluctant to commit American military forces, even in the form of missile strikes, to another 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 United Nations 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.
Mr. Obama spoke on 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." Mr. Obama had spoken on Saturday to the British prime minister, David Cameron.
Those discussions, officials said, were aimed at gauging whether Britain and France are on the same page as the United States, given that they could be part of an international coalition carrying out strikes against Syria.
While officials said the United States would still hold consultations at the United Nations, they made it clear that the United Nations was not the only avenue for taking action 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 an agreement to allow access beginning on 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 Mr. 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 on 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 provoked American military action.
The Syrian government has denied using chemical weapons as part of a civil war that has left more than 100,000 people dead. On 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 believe they have the supplies or ability to do so.
In a response that suggested concern about a possible imminent American attack, Moscow welcomed Syria's acceptance of inspections and cautioned against a rush to judgment. A spokesman for the foreign ministry, Aleksandr 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 United Nations inspections.
"In these conditions, we again resolutely call on all those who are trying to impose the results of the U.N. investigations and who say that armed actions against Syria is possible to show common sense and avoid tragic mistakes," Mr. Lukashevich said in a statement released on Sunday evening on the ministry's Web site.
Syria warned that any American military action would "create a ball of fire that will inflame the Middle East," according to The Associated Press. And Iranian state news media quoted the Tehran government as saying that any intervention by Washington would have severe consequences.
Complicating the task of the United Nations team is the fact that the attack on Wednesday took place in an area that remains behind rebel lines. 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 chemical weapons had been use, not to assign responsibility for their deployment.
Israel sharpened its message on 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," he 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 is now 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," Mr. 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."
In Washington, several lawmakers said on Sunday that they now expected limited military action to punish Syria or deter chemical attacks.
But lawmakers who appeared on Sunday talks show said it would be reckless to insert ground troops into a war in a region already in turmoil, and there was a general call for any action to be taken under the broadest possible international auspices.
"I hope the president, as soon as we get back to Washington, will ask for authorization from Congress to do something in a very surgical and proportional way," Senator Bob Corker of Tennessee, the top Republican on the Senate Foreign Relations Committee, said on "Fox News Sunday."
But Representative Eliot Engel of New York, the ranking Democrat on the House Foreign Affairs Committee, said the situation might be too urgent to wait for Congress, which does not return from its summer recess until Sept. 9. Mr. Engel suggested that there were many options for air attacks launched from outside Syrian airspace.
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.
The group said it could not confirm what substances caused the symptoms it reported or who was responsible for the attack, but its report appeared to lend credibility to other accounts by witnesses and to the opposition's estimates of the number of people killed.
Scott Shane reported from Washington, and Ben Hubbard from Beirut, Lebanon. Reporting was contributed by Isabel Kershner from Jerusalem, Steven Lee Myers from Moscow, and Michael R. Gordon, Mark Landler and Brian Knowlton from Washington.
This article originally appeared in The New York Times.