The early intelligence finding could increase pressure for action by President Barack Obama, who made clear that he planned to tread cautiously even as his aides sought to narrow their differences in debate over possible military responses to the Syrian government.
The sources, speaking on condition of anonymity, cautioned that the assessment was preliminary and, at this stage, they were still seeking conclusive proof, which could take days, weeks or even longer to gather.
But with a mounting international outcry over the apparent mass poisoning of hundreds of people, the issue appeared to have taken on a sense of urgency for the Obama administration.
In his first public comments since Wednesday's attack in the Damascus suburbs, Obama called the incident a "big event of grave concern" and one that demanded U.S. attention, but said he was in no rush to get war-weary Americans "mired" in another Middle East conflict.
Obama's wary response, which underscored a deep reluctance by Washington to intervene in Syria's 2-1/2-year-old civil war, came as senior U.S. officials weighed choices ranging from increased international sanctions to the use of force, including possible air strikes on Assad's forces, administration sources said.
A meeting of members of Obama's National Security Council, the Pentagon, State Department and intelligence agencies was held at the White House late on Thursday, but made no decisions on what to recommend, officials said.
With further talks planned as early as this weekend, a senior U.S. defense official said a decision on a course of action could come soon. But it appeared unlikely any military response would take place without extensive consultation with allies and further review of U.S. intelligence about the attack.
One U.S. official acknowledged that the participants aired "differing viewpoints," but pushed back against the notion that the administration, whose Syria policymaking has been marked by internal dissent in the past, was sharply divided on a response.
"It's not like people were screaming at each other," the official said.
International powers - including Russia, which has long shielded Assad from U.N. action - have urged Assad to cooperate with a U.N. inspection team that arrived on Sunday to pursue earlier allegations of chemical weapons attacks.
White House spokesman Josh Earnest said there was "some evidence" of chemical weapons use in the latest incident, but stopped short of saying an official conclusion was reached.
The Syrian government denies being responsible and has in the past accused rebels of using chemical weapons, an allegation that Western officials have dismissed.
CROSSING THE 'RED LINE'
While the preliminary U.S. assessment was that Assad loyalists carried out Wednesday's attack with high-level authorization, one U.S. source closely monitoring events in the region said it was also possible that a local commander decided on his own to use gas to clear the way for a ground assault.
"What we've seen indicates that this is clearly a big event, of grave concern," Obama said in an interview on CNN's "New Day" program that aired on Friday, as anti-Assad rebels braved the front lines around Damascus to smuggle tissue samples to U.N. inspectors from victims of Wednesday's apparent mass poisoning.
Asked about his comment - made a year and a day before the toxic fumes hit sleeping residents of rebel-held Damascus suburbs - that chemical weapons would be a 'red line' for the United States, Obama expressed caution.
"If the U.S. goes in and attacks another country without a U.N. mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it," Obama said. "The notion that the U.S. can somehow solve what is a sectarian complex problem inside of Syria sometimes is overstated."
At Thursday's White House meeting, which lasted more than three hours, Obama's aides had a "robust discussion" of the diplomatic and military options available to the president, U.S. officials said.
Among the military options under consideration are targeted missile strikes on Syrian units believed responsible for chemical attacks or on Assad's air force and ballistic missile sites, U.S. officials said. Such strikes could be launched from U.S. ships or combat aircraft capable of firing missiles from outside Syrian airspace, thereby avoiding Syrian air defenses.
Seen as more risky - and unlikely - would be a sustained air assault, such as the one conducted in Libya in 2011.
Secretary of State John Kerry, who took part in Thursday's meeting by secure video link, advocated the use of air strikes in White House meetings in early June preceding an announcement of military aid to the rebels, a person familiar with the talks said. Joint Chiefs Chairman General Martin Dempsey argued that such a mission would be complex and costly.
The White House on Friday reiterated Obama's position that he did not intend to put "boots on the ground" in Syria, and an administration official said Thursday's meeting also steered clear of the idea of enforcing a "no-fly" zone there.
Another possibility would be to authorize sending heavier U.S. weaponry, such as shoulder-fired anti-aircraft rockets, to the rebels in addition to lighter arms approved in June. But even those limited supplies have yet to start flowing to the rebels.
The top Democrat on the House of Representatives Foreign Affairs Committee urged Obama on Friday to order air strikes against Assad's government.
Representative Eliot Engel cited Obama's statement that the use of chemical weapons by Assad's forces would cross a "red line" and cause the United States to act to halt such violations of international law.
"If we, in concert with our allies, do not respond to Assad's murderous uses of weapons of mass destruction, malevolent countries and bad actors around the world will see a green light where one was never intended," Engel wrote in a letter to Obama and obtained by Reuters.
'CREDIBILITY ISSUE'
With Obama's international prestige seen on the line, a former senior U.S. official said the suspected chemical attack was likely to prompt Obama to use limited force, but he did not expect him to try to topple Assad.
"They will feel obliged to do something because … the credibility issue is very high here," the former official said, speaking on condition of anonymity. "Something like a finite use of stand-off force is quite possible here."
Obama's failure to confront Assad with the serious consequences he has long threatened would likely reinforce a global perception of a president preoccupied with domestic matters and unwilling to act decisively in the volatile Middle East, a picture already set by his mixed response to the crisis in Egypt.
The consensus in Washington and allied capitals is that a concerted international response can only succeed if the United States takes the lead.
But Obama has shown no appetite for intervention. Polls by Reuters/Ipsos and others have shown that most Americans, weary of wars in Iraq and Afghanistan, are increasingly aware of the Syria conflict but remain opposed to U.S. involvement there.
(Additional reporting by Arshad Mohammed, Roberta Rampton, Jeff Mason, Andrea Shalal-Esa, Patricia Zengerle; Editing by Vicki Allen, Eric Beech and Peter Cooney)
