The U.S.-backed drive by Iraqi Kurds to wrest the northern town of Sinjar and a slice of strategic highway from Islamic State could give new momentum to the Obama administration's strategy to defeat the extremist group, U.S. officials and analysts said on Thursday.

If the Kurdish forces can hold Highway 47 in Sinjar, they would cut the main supply route between Islamic State's forces in northern and central Iraq and their main stronghold of Raqqa in the north of neighbouring Syria. That, the officials and analysts said, would make it harder for the group to move reinforcements and supplies back and forth across the border.

The long-awaited Sinjar offensive is one of several signs, the officials argued, that U.S. President Barack Obama's much-criticized, slow-burn strategy against Islamic State in both Iraq and Syria is starting to produce results.

It coincides with a renewed push by Washington to find a diplomatic solution to the Syrian civil war, which U.S. Secretary of State John Kerry said on Thursday would hinge partly on perceptions of which military forces were gaining the upper hand.

Elsewhere, Iraqi forces are encircling the Islamic State-held city of Ramadi after capturing the key petroleum refinery city of Bayji. U.S.-backed rebels have made advances against the group in northeastern Syria, where Obama last month decided to deploy U.S. special operations forces as advisers.

"They are under a lot of pressure on a lot of fronts," said Jeffrey White, a former senior Defence Intelligence Agency analyst. "They will face tough decisions on how to allocate forces," said White, a fellow at the Washington Institute for Near East Policy.

White and two U.S. officials noted that while Sinjar's fall would be an important success, there still would be a long way to go to crush the Islamic State, which has shown an ability to adapt to setbacks - and lash out elsewhere when attacked during the 14-month-long U.S.-led campaign against it.

"You're seeing a kind of a war of attrition start to play in here," said one U.S. official, speaking on condition of anonymity. But the Islamic State still has "tremendous assets" and "incredible determination to carry this on to the death."

Any attempt to retake Mosul, Iraq's second-largest city, from the Islamic State remains months away, the officials said.

Obama's strategy, which he unveiled last year after Islamic State fighters charged out of their sanctuary in Syria and seized huge swaths of Iraq, originally called for first isolating and defeating the group in Iraq.

The United States forged an international coalition that has been staging air strikes against the Islamic State and helping to re-build and advise the Iraqi army, which all but collapsed in the face of the Islamist onslaught, and the Kurdish Peshmerga.

Progress, however, has been slow. Iraqi forces, including Iran-backed Shiite Muslim militias, have encountered fierce resistance in their efforts to claw back territory, especially in Sunni-dominated Anbar Province.

On Syria, where Russia and Iran have increased their military support for Assad, U.S. lawmakers and others have accused the administration of lacking a coherent approach for dealing with Islamic State. The Pentagon tried but failed to cobble together a moderate opposition force that only would fight the Islamic State, but not Assad's forces.

Obama announced late last month that up to 50 U.S. special operations troops would be deployed in northern Syria from this month to advise opposition forces, shifting a year-old strategy hinged on battling Islamic State without putting American "boots on the ground."

The strategy change also includes positioning more U.S. jets in Turkey to expand American air strikes as Syrian Kurds, Arabs and other opposition fighters increase pressure on Raqqa.


Kerry, in a forceful defence of the administration's strategy, said on Thursday that the fight against Islamic State would be a "multi-year effort."

"There is increasing evidence in both Iraq and Syria that (Islamic State) can be defeated, even routed, when faced by the combination of coalition air strikes and effective partners on the ground," Kerry said, before heading to a new round of Syrian peace talks in Vienna this weekend

In advance of the Kurds' offensive the U.S.-led coalition carried out 90 air strikes around Sinjar since 1 November, according to the Pentagon. More than 150 square km (58 square miles) have been seized from the ultra-hardline Sunni group and dozens of bodies of its fighters were left behind in a retreat from parts of Sinjar, the Kurdistan Regional Security Council said.

A handful of U.S. military advisors are working with Kurdish Peshmerga fighters on and near Sinjar mountain, but are positioned well back from the fighting, a U.S. military spokesman said.

It will "discomfit (Islamic State) for some time," said Kenneth Pollack, a former CIA and White House official now at the Brookings Institution think tank. "This, in and of itself, is unlikely to be a major blow to them."

If they can hold the highway, the Kurds will make it harder for Islamic State to resupply its fighters in Iraq, including the militants holding Ramadi, the Anbar provincial capital.

There are work-arounds for the group, officials and analysts said, but they involve transiting desert tracks to the south of the main highway.

"They are pretty smart about trading space in order not to allow their forces to be destroyed," said White. "They will probably come up with a well thought-out response."