The White House has concluded that there is "very little doubt" that the Syrian regime used chemical weapons against its own people in an attack last week, increasing the chances of a U.S. military strike.
The chemical weapons assessment is based on a variety of evidence and represents a broad consensus, according to a statement from a senior administration official.
The official requested anonymity; deliberations are ongoing and no decision has been reached about what to do.
Syria agreed Sunday to allow a United Nations investigation into the alleged chemical weapons attack that killed hundreds. The White House official said the offer comes too late.
President Obama was presented with a range of military options as he huddled with his national security team on Saturday to discuss Syria.
Military analysts say the likeliest option would be a punitive strike designed to send a message to the regime of Bashar Assad but that would not be designed to decapitate the regime or dramatically alter the course of the civil war raging there.
"Behavior modification would be the main objective rather than decisively shifting the situation on the ground or removing the regime," said Jeffrey White, an analyst at the Washington Institute for Near East Policy and a former Defense Intelligence Agency official.
Administration officials have been wary of any military intervention that would draw the United States into a lengthy commitment, and they have also expressed concerns that the collapse of the regime might lead to a failed state or the emergence of al-Qaeda affiliates.
Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, has expressed concerns that no single rebel group is capable yet of taking over and representing U.S. interests.
Military analysts say U.S. and allied forces could strike targets in Syria with cruise missiles launched from ships or aircraft in the region without having to penetrate Syria's air defense system.
"Any strike is going to be standoff," said Elizabeth O'Bagy, an analyst with the Institute of the Study of War.
Syria has an extensive air defense system and, although it is not up-to-date, any attack through its airspace would carry high risks. By contrast, cruise missiles can be launched from a safe distance.
If the United States decides to take military action, it will likely do so with allies. Britain and France have both advocated a strong response if it is proven that Assad's regime has used chemical weapons.
On Sunday, Obama spoke by telephone with French President Francois Hollande and discussed possible responses to the alleged chemical weapons attack, the White House said.
The White House conclusion about the use of chemical weapons raises the stakes in a civil war that has already killed more than 100,000 Syrians in more than two years of fighting. "I see this as a defining moment for the administration," White said.
The White House has said the use of chemical weapons would be a red line, triggering a U.S. response. Any U.S. military intervention, even a limited one, would likely give renewed hope to rebel fighters, who have suffered setbacks in recent months.
Obama should follow through on his commitment to take action if that red line was crossed, said Najib Ghadbian, the Syrian National Coalition's representative to the USA.
The White House said its assessment that the regime used chemical weapons in an attack Aug. 21 was built on extensive evidence.
"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," according to the statement from the senior administration official.
The statement also said Syria's offer to allow U.N. inspectors is "too late to be credible" because any evidence has been ruined by the regime's persistant shelling.
Analysts also say that getting full access to the site would be difficult because of fighting in the area.
Collecting evidence from the site would be "extremely challenging" and probably unsafe for the U.N. inspectors, said Hamish de Bretton-Gordon, a former commander of the chemical, biological and nuclear counterterrorism unit at the United Kingdom's Ministry of Defense.