WASHINGTON — Pentagon officials have warned for months that Islamic State, a Sunni extremist group that President Obama initially dismissed as a “JV team,” is nimble, aggressive and unlike any previous terrorist group.

On Wednesday, the militants’ latest battlefield successes forced the White House to tack sharply and announce deployment of an additional 450 U.S. military advisers and support troops to help bolster Iraq’s beleaguered security forces, including local Sunni tribes.

The boost in troops, the first since November, will expand the U.S. footprint to a fifth base, close to insurgent strongholds in Anbar province, even though at least one of the four training bases where U.S. trainers already operate lacks willing Iraqi recruits.

“This decision does not represent a change in mission,” Secretary of Defense Ashton Carter said in a statement, noting that U.S. troops still will be barred from conducting offensive ground combat.

But the new focus on breaking the extremists’ grip in Anbar marks a major shift in U.S. strategy and puts American troops considerably closer to the front lines.

The militants’ surprise capture last month of Ramadi, capital of Anbar, prompted a reappraisal of Pentagon planning. A fear that Islamic State fighters would entrench themselves in the city and use it as a launching pad for attacks on Baghdad, an hour’s drive away, proved a factor.

But so did a White House recognition that the military approach that has evolved over the last year, a combination of intense coalition airstrikes and hit-or-miss Iraqi ground assaults, isn’t close to meeting Obama’s objective of pushing Islamic State out of Iraq.

In recent months, the Pentagon had highlighted efforts to retake the strategic northern city of Mosul, even telling reporters this spring that a U.S.-backed ground offensive was all but imminent.

That ambitious plan has been shelved indefinitely. Mosul, a city of 1 million, fell to the insurgents a year ago Wednesday and remains the largest city under strict Islamic State control and serves as the Iraqi capital of the group’s self-declared caliphate.

Instead, U.S. officials are seeking to rekindle the so-called 2006-08 Anbar Awakening. Back then, Sunni tribal fighters paid and armed by the U.S. took on and ultimately helped defeat the al-Qaida militants who had fueled a vicious sectarian war after the 2003 U.S.-led invasion.

Col. Steve Warren, the Pentagon spokesman, cited that model Wednesday, saying the goal is to enlist the Sunni militias to help retake Ramadi and the nearby militant stronghold of Fallujah.