ASPEN, Colo. -- A senior American intelligence official on Saturday warned that the Syrian conflict could last "many, many months to multiple years," and described a situation that would most likely worsen regardless of whether the Syrian leader, President Bashar al-Assad, fell.
The comments by David R. Shedd, the deputy director of the Defense Intelligence Agency, were one of the strongest public warnings about how the civil war in Syria has deteriorated, and he seemed to imply that the response from the United States and its allies had so far been lacking.
Mr. Shedd suggested that in addition to strengthening the more secular groups of the fractious Syrian opposition -- which the Obama administration has promised to arm with weapons and ammunition -- the West would have to directly confront more radical Islamist elements. But he did not say how that could be accomplished.
"The reality is that, left unchecked, they will become bigger," Mr. Shedd told the Aspen Security Forum, an annual meeting on security issues. "Over the last two years they've grown in size, they've grown in capability, and ruthlessly have grown in effectiveness."
At the forum, of which The New York Times is a sponsor, Mr. Shedd described two different scenarios for Syria's future, both of which he said portended far more violence and killing.
"If Bashar Assad were to succeed, he will be a more ruthless leader who will live with a legacy of tens of thousands of his civilians killed under him," he said. President Obama declared in mid-2011 that Mr. Assad had to leave office.
Mr. Shedd outlined an equally grim portrait of a spreading Sunni-Shiite sectarian conflict if Mr. Assad's government fell or he was killed.
"If he loses and goes to an enclave inside there, I think there will be ongoing civil war for years to come," he said, noting that more radical elements like the Nusra Front would fight to control parts of the country. "They will fight for that space. They're there for the long haul."
Mr. Shedd offered a sobering assessment of America's ability to draw distinctions among an opposition that he said numbered about 1,200 groups.
After months of internal debate, the Obama administration in June announced a plan to provide direct military support to the Syrian rebels, but so far no arms have arrived.
The effort to support the opposition has also been hampered by the inability of the United States and various Arab countries -- including Saudi Arabia, Qatar and the United Arab Emirates -- to agree on how quickly to act, which opposition groups to support, and which weapons to give them.
Mr. Shedd, a 31-year intelligence veteran, seemed to suggest that modest interventions were unlikely to make a significant difference at a time when Mr. Assad's army has been reclaiming territory on the battlefield, with the support of Iran, Russia and Hezbollah, and when the opposition is bitterly divided, while among the rebels the Islamists are resurgent.
"My concern is that it could go on for a long time," Mr. Shedd said, voicing concern that the civilian casualties, refugee flows and internal dislocation would increase. "It is in large measure a stalemate."
Mr. Shedd said he was particularly concerned that the Syrian revolution, unlike the other Arab uprisings, was far more likely to explode than implode, and that Jordan and Iraq would be caught in the conflict and instability.
His publicly expressed concerns about Jordan contrasted with the administration's usual, almost ritualized declarations of confidence that King Abdullah II of Jordan could emerge with his country, one of America's strongest allies in the region, intact.
Mr. Shedd also played down reported dispute between the Nusra Front and Al Qaeda's arm in Iraq over which one controls the more radical elements in Syria. But he expressed concerns that the Qaeda branch would strengthen its position inside Iraq, after having been largely decimated by the American troop surge there in 2007.
"Al Qaeda Iraq will emerge stronger as a result of its experience inside of Syria," he said.
This article originally appeared in The New York Times.