(Adds no deadline set for U.S. intelligence, high-level meetings, Kerry talks)
By Matt Spetalnick and Lesley Wroughton
WASHINGTON, Aug 22 (Reuters) - With his international credibility seen increasingly on the line, President Barack Obama on Thursday faced growing calls at home and abroad for forceful action against the Syrian government over accusations it carried out a massive new deadly chemical weapons attack.
While the White House said it was "appalled" by reports of hundreds of people gassed on Wednesday, it made clear any U.S. response would await confirmation of a chemical attack and again demanded that Syrian President Bashar al-Assad give U.N. inspectors immediate access to the site near Damascus.
The Obama administration's cautious response underscored a deep reluctance by Washington to intervene in Syria since the country's civil war erupted 2 1/2 years ago.
But, reflecting the pressures Obama could face in coming days, a U.S. official familiar with initial intelligence assessments said the attack appeared to be the deliberate work of the Assad government. It was "the regime acting as a regime," the official said.
If allegations of a large-scale chemical attack are verified - Syria's government has denied them - Obama will surely face calls to move more aggressively, possibly even with military force, in retaliation for repeated violations of U.S. "red lines."
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 indicated that Americans are increasingly aware of the conflict in Syria, but as the news has worsened, opposition to intervention may actually be growing.
Despite that, pressure was mounting as horrific photos and videos of alleged chemical weapons victims spread across the Internet.
French Foreign Minister Laurent Fabius said world powers must respond with force if it is proved that Syria's government was responsible for the deadliest chemical attack on civilians in a quarter-century.
But Fabius stressed there was no question of sending in troops and his remarks appeared to be an effort to prod Washington and others to action.
Israel, a longtime foe of Assad, said it believed Syrian forces had used chemical weapons in the killing of hundreds of people in the rebel-held suburbs of Damascus, and it accused the world of turning a blind eye to such attacks.
NO 'CONCLUSIVE' DETERMINATION BY U.S.
State Department spokeswoman Jen Psaki said the United States had not "conclusively" determined that chemical weapons were employed but that Obama had directed the U.S. intelligence community to urgently gather information to verify the reports from the Syrian opposition.
But another U.S. official said intelligence agencies were not given a deadline and would take the time needed to "reach a conclusion with confidence." The administration held high-level meetings to deliberate on Syria policy, the official said.
Psaki said Assad's use of chemical arms would be an "outrageous and flagrant escalation," but stopped short of saying what options were under consideration.
"If we find these reports are true, then we will feel that this has significantly expanded the escalation of the use of chemical weapons by the Syrian regime," she said. "The president, the national security team, would certainly have decisions to make, and they have a range of options to decide between."
Secretary of State John Kerry spoke about the crisis on Thursday with U.N. Secretary-General Ban Ki-moon, EU foreign policy chief Catherine Ashton and Kerry's counterparts from France, Jordan, Qatar and Turkey.
Western diplomats said their efforts for now were focused on persuading the Syrian government to allow the U.N. inspection team, already in Damascus, to the site of the alleged attacks.
"We will all have to be clear that there is a price to pay for not letting the team in," one diplomat said, without elaborating. Russia's shielding of Syria in the U.N. Security Council, where Moscow holds a veto, could blunt any significant measures by the world body.
Psaki acknowledged that Obama's "red line" against Syrian chemical weapons use "was crossed a couple of months ago."
Obama's decision in June to begin arming Syrian rebels was linked to a U.S. intelligence finding that Assad's forces had used chemical weapons in several small-scale attacks. But even the limited arms supplies authorized by the president have yet to start flowing.
The latest Syria controversy has added to a growing perception of foreign policy troubles for Obama early in his second term. He is facing criticism for his inability to restrain Egypt's generals in their violent crackdown on Islamists and for failing to persuade Russia to extradite fugitive former spy agency contractor Edward Snowden.
On Syria, White House officials have cited several factors to explain their caution: a fractious anti-Assad rebel movement, lack of direct U.S. security interests, and the high cost of intervention.
Pressing ahead with a bus tour in the U.S. Northeast to promote his economic agenda, Obama made no public mention of Syria. "The fact that we are doing this bus tour is an indication that the president has his priorities straight while he continues to monitor what is an increasingly tragic situation in Syria," White House spokesman Josh Earnest told reporters.
Critics said the Obama administration's international credibility had already been damaged by his handling of the Syria conflict but that it would be even worse if chemical weapons use were confirmed and Washington failed to act.
"You don't want to lay down a red line and not enforce it," said Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy, who called the Syria crisis the "biggest black mark" on Obama's foreign policy record.
Fred Hof, a former senior State Department adviser on Syria who is now at the Atlantic Council think tank, wrote on Thursday, "The Assad regime, Iran, its Lebanese militia, and Russia have taken the measure of the United States in the Syrian crisis and have concluded they can win."
In the U.S. Congress, Republican Senator John McCain said the "credible reports" from Syria on chemical weapons use by Assad's forces "should shock our collective conscience."
"It is long past time for the United States and our friends and allies to respond to Assad's continuing mass atrocities in Syria with decisive actions, including limited military strikes to degrade Assad's air power and ballistic missile capabilities," McCain, a harsh critic of Obama's Syria policy, said in a statement.
Democratic Senator Robert Menendez, chairman of the Senate Foreign Relations Committee, said he was "shocked and deeply concerned" about the reported chemical weapons attack, but he stopped short of calling for military action.
The top U.S. military officer, Joint Chiefs chairman General Martin Dempsey, canceled a planned press briefing on security issues on Thursday. Dempsey, in letters to lawmakers, has made clear the U.S. armed forces judge that intervention in Syria would be costly and have an uncertain outcome.
Many members of Congress have echoed the administration's concerns about involvement in Syria, worried that weapons sent to anti-Assad rebels could end up in the hands of Islamists.
Unless U.N. inspectors are able to conduct an investigation, it could take some time for U.S. officials to sift through photographs, video and intelligence to determine whether the Syrian opposition's reports are credible.
An earlier U.S. investigation of alleged Syrian chemical weapons use took months to conclude that Assad's forces had used small amounts of sarin gas in attacks during the previous year. (Additional reporting by Tabassum Zakaria, Roberta Rampton, Jeff Mason, Susan Cornwell, Patricia Zengerle and Mark Hosenball; Editing by Warren Strobel and Peter Cooney)