WASHINGTON (Reuters) - Secretary of State John Kerry’s public assertions that moderate Syrian opposition groups are growing in influence appear to be at odds with estimates by U.S. and European intelligence sources and nongovernmental experts, who say Islamic extremists remain by far the fiercest and best-organized rebel elements.

U.S. Secretary of State John Kerry testifies at a U.S. House Foreign Affairs Committee hearing on Syria on Capitol Hill in Washington, September 4, 2013. REUTERS/Jason Reed

At congressional hearings this week, while making the case for President Barack Obama’s plan for limited military action in Syria, Kerry asserted that the armed opposition to Syrian President Bashar al-Assad “has increasingly become more defined by its moderation, more defined by the breadth of its membership, and more defined by its adherence to some, you know, democratic process and to an all-inclusive, minority-protecting constitution.

“And the opposition is getting stronger by the day,” Kerry told the Senate Foreign Relations Committee on Tuesday.

U.S. and allied intelligence sources and private experts on the Syrian conflict suggest that assessment is optimistic.

While the radical Islamists among the rebels may not be numerically superior to more moderate fighters, they say, Islamist groups like the al Qaeda-aligned Nusra Front are better organized, armed and trained.

Kerry’s remarks represented a change in tone by the Obama administration, which for more than two years has been wary of sending U.S. arms to the rebels, citing fears they could fall into radical Islamists’ hands.

As recently as late July, at a security conference in Aspen, Colorado, the deputy director of the Pentagon’s Defense Intelligence Agency, David Shedd, estimated that there were at least 1,200 different Syrian rebel groups and that Islamic extremists, notably the Nusra Front, were well-placed to expand their influence.

“Left unchecked, I’m very concerned that the most radical elements will take over larger segments” of the opposition groups, Shedd said. He added that the conflict could drag on anywhere “from many, many months to multiple years” and that a prolonged stalemate could leave open parts of Syria to potential control by radical fighters.

U.S. and allied intelligence sources said that such assessments have not changed.

A spokeswoman at the State Department said Kerry’s remarks reflect the department’s position, adding that the opposition had “taken steps over the past months to coalesce, including electing leaders.”

GREATER NUMBERS, LESS STRENGTH?

Experts agree that the Nusra Front, an offshoot of the group al Qaeda in Iraq, is among the most effective forces in Syria.

In a second hearing on Wednesday, Kerry was challenged by Representative Michael McCaul, Texas Republican.

“Who are the rebel forces? Who are they? I ask that in my briefings all the time,” McCaul said. “And every time I get briefed on this it gets worse and worse, because the majority now of these rebel forces - and I say majority now - are radical Islamists pouring in from all over the world.”

Kerry replied: “I just don’t agree that a majority are al Qaeda and the bad guys. That’s not true. There are about 70,000 to 100,000 oppositionists ... Maybe 15 percent to 25 percent might be in one group or another who are what we would deem to be bad guys.

“There is a real moderate opposition that exists. General Idriss is running the military arm of that,” Kerry continued, referring to General Salim Idriss, head of the rebel Free Syrian Army. Increasingly, he said, Saudi Arabia and other Gulf states are funneling assistance through Idriss.

Kerry cited an article by Elizabeth O’Bagy, an analyst with the Institute for the Study of War think tank, in which she wrote that Islamic extremist factions are not “spearheading the fight against the Syrian government,” but rather that the struggle is being led by “moderate opposition forces.”

Several leading lawmakers, including Senator John McCain, Arizona Republican, also have said there is a viable moderate opposition in Syria that Washington should support.

U.S. intelligence sources do not dispute that Islamic extremists are in the minority on the battlefield.

“Most of the groups battling against Assad are composed of Islamist fighters, but only a small minority could accurately be characterized as extremist,” one U.S. official said, speaking on condition of anonymity.

But a second official, who also asked not to be named, said moderate rebels may have lost strength rather than gained it in recent months. Due to their relative lack of weapons and organization, they are beginning to make alliances with better-armed Islamic radicals, whom they see pursuing more effective actions against Assad’s forces, the official said.

Paul Pillar, who retired in 2005 as the U.S. intelligence community’s top Middle East analyst, said he believed the Obama Administration was walking a fine line, trying to calculate how to punish Assad’s government for allegedly using chemical weapons while not bolstering the strength of religious militant rebels.

“In a hard-fought civil war, especially one without a single well-organized opposition movement, success goes to the most ruthless and dedicated elements, which also tend to be the most extreme in their views. We are seeing such a process in Syria today,” Pillar said.

An authorization to use military force approved on Wednesday by the Senate Foreign Relations Committee states that U.S. policy in Syria includes “upgrading the lethal and non-lethal military capabilities of vetted elements of Syrian opposition forces.”

‘CHOOSING ONE AMONG MANY SIDES’

Top U.S. intelligence and military officials have recently offered bleak public evaluations of the relative strengths of moderate and religious extremist Syrian rebels.

In an August 19 letter to Representative Eliot Engel, obtained by Reuters, the chairman of the Joint Chiefs of Staff, General Martin Dempsey, warned: “Syria is not about choosing between two sides but rather about choosing one among many sides.

“It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favor,” Dempsey wrote. “Today they are not.”

A European security official with experience in the region said that extremist rebel factions were so strong and well-organized in the north and west of Syria that they were setting up their own public services and trying to create an Islamic ministate along the Iraqi border.

By contrast, the official said, more moderate rebel factions predominate in the east of Syria and along its southern border with Jordan but have largely devolved into “gangs” whose leaders are more interested in operating local rackets and enriching themselves than in forming a larger alliance that could more effectively oppose Assad’s government.

“I’ve heard that there are moderate groups out there we could, in theory, support,” said Joshua Foust, a former U.S. intelligence analyst who now writes about foreign policy.

“But I’ve heard from those same people and my own contacts within (U.S. intelligence) that the scary people are displacing more and more moderate groups. Basically, the jihadists are setting up governance and community councils while the moderates exhaust themselves doing the heavy fighting,” Foust said.

As anecdotal evidence, Foust cited a recent report that on August 22, four out of five commanders of the moderate Supreme Military Council had threatened to resign and work “with all forces fighting in Syria.”

A video on YouTube shows the rebel commander who made this announcement. He is seated in front of an Islamic extremist flag, next to a bearded cleric clad in the religious dress of a Salafist militant.