WASHINGTON -- American intelligence agencies have concluded that despite billions of dollars spent and more than 10,000 extremist fighters killed, the Islamic State group is fundamentally no weaker than it was when the U.S.-led bombing campaign began a year ago.

U.S. military commanders on the ground aren't disputing the assessment, but they point to an upcoming effort to clear the important Sunni city of Ramadi, which fell to the militants in May, as a crucial milestone.

The battle for Ramadi, expected over the next few months, "promises to test the mettle" of Iraq's security forces, Marine Corps Brig. Gen. Kevin J. Killea, who is helping run the U.S.-led coalition effort in Iraq, told reporters at the Pentagon in a video briefing from the region.

The U.S.-led military campaign has put the Islamic State group on defence, Killea said, adding, "There is progress." Witnesses on the ground say the airstrikes and Kurdish ground actions are squeezing the militants in northern Syria, particularly in their self-proclaimed capital in Raqqa.

But U.S. intelligence agencies see the overall situation as a strategic stalemate: The Islamic State remains a well-funded extremist army able to replenish its ranks with foreign jihadis as quickly as the U.S. can eliminate them. Meanwhile, the group has expanded to other countries, including Libya, Egypt's Sinai Peninsula and Afghanistan.

The assessments by the CIA, the Defence Intelligence Agency and others appear to contradict the optimistic line taken by the Obama administration's special envoy, retired Gen. John Allen, who told a forum in Aspen, Colorado, last week that "ISIS is losing" in Iraq and Syria. The intelligence was described by officials who would not be named because they were not authorized to discuss it publicly.

"We've seen no meaningful degradation in their numbers," a defence official said, citing intelligence estimates that put the group's total strength at between 20,000 and 30,000, the same estimate as last August, when the airstrikes began.

The Islamic State's staying power raises questions about the administration's approach to the threat that the group poses to the U.S. and its allies. Although officials do not believe it is planning complex attacks on the West from its territory, the group's call to Western Muslims to kill at home has become a serious problem, FBI Director James Comey and other officials say.

Yet under the Obama administration's campaign of bombing and training, which prohibits American troops from accompanying fighters into combat or directing airstrikes from the ground, it could take a decade or more to drive the Islamic State from its safe havens, analysts say. The administration is adamant that it will commit no U.S. ground troops to the fight despite calls from some in Congress to do so.

The U.S.-led coalition and its Syrian and Kurdish allies have made some inroads. The Islamic State has lost 9.4 per cent of its territory in the first six months of 2015, according to an analysis by the conflict monitoring group IHS.

A Delta Force raid in Syria that killed Islamic State financier Abu Sayyaf in May also has resulted in a well of intelligence about the group's structure and finances, U.S. officials say. His wife, held in Iraq, has been co-operating with interrogators.

Syrian Kurdish fighters and their allies have wrested most of the northern Syria border from the Islamic State group, and the plan announced this week for a U.S.-Turkish "safe zone" is expected to cement those gains.

In Raqqa, U.S. coalition bombs pound the group's positions and target its leaders with increasing regularity. The militants' movements have been hampered by strikes against bridges, and some fighters are sending their families away to safer ground.

But American intelligence officials and other experts say the Islamic State is in no danger of being defeated any time soon.

"The pressure on Raqqa is significant ... but looking at the overall picture, ISIS is mostly in the same place," said Harleen Gambhir, a counterterrorism analyst at Institute for the Study of War, a Washington think-tank.