The Obama administration backed away Friday from a blighted effort to train Syrian rebels to fight the Islamic State, as the Pentagon announced plans instead to provide direct aid to existing rebel units it believes have better odds of succeeding against the militants.

The decision is a recognition of the repeated failures of a program begun early this year. Most recently, newly trained fighters were attacked by rival forces ­engaged in a largely separate fight against Syrian President Bashar al-Assad. One U.S.-trained unit decided to hand over ­Washington-provided equipment to the local al-Qaeda affiliate.

U.S. officials hope the revised plan will help Arab forces, allied with Syrian Kurdish fighters, replicate the success that the Kurds have had against the Islamic State in northeastern Syria and eventually isolate the group in Raqqa, its de facto Syrian capital.

“What we’re really trying to do here is build on what has worked for us and learn from some of the things that have been a lot more challenging,” Christine E. Wormuth, undersecretary of defense for policy, told reporters.

Wormuth said the training program launched earlier this year has not been canceled but was put on “operational pause” and might be resumed in the future.

The move marks an expansion of U.S. involvement in Syria’s protracted ground war and could expose the Obama administration to greater risks if weapons provided to a wider array of rebel units go astray, or if U.S.-backed fighters come under attack from forces loyal to Assad and his allies.

For much of the past year, U.S. military officials struggled to get the Pentagon’s original training program up and running. Under the initial plan, the military vetted individual Syrians, took them out of the country and put them through a weeks-long training course in Turkey or Jordan.

But qualified candidates have been hard to find — especially given a U.S. requirement that they fight only the Islamic State and not the Assad regime — and even harder to track once they have returned to Syria. Fewer than 200 fighters have been trained.

Under the new plan, leaders of groups already battling the Islamic State undergo vetting and receive a crash course in human rights and combat communications. Many of them have already received that training outside Syria, officials said.

Eventually the Pentagon plans to provide ammunition and basic weapons to those leaders’ fighters and would carry out airstrikes on targets identified by those units. Most, if not all, of the rank and file would be neither vetted nor trained by the United States.

The strategy overhaul, approved by President Obama last week, comes as Washington scrambles to adjust to Moscow’s entry into the Syrian war. Russian planes and forces are now backing an offensive by troops loyal to Assad against anti-regime rebels, complicating the United States’ own air operations over Syria.

Map: What a year of Islamic State terror looks like

It also occurs as officials explore ways to reinvigorate a parallel effort in Iraq, where local forces have been unable to dislodge the Islamic State from key urban areas.

The new approach appears to have originated with the victory of Syrian Kurdish forces, backed by U.S. airstrikes, against the Islamic State in the northern Syrian border town of Kobane early this year. American officials were impressed by the Kurdish units’ tenacity and their ability to capi­tal­ize on U.S. air power.

Some of the rebel units that could receive new U.S. support are part of what the administration calls the Syrian Arab Coalition, located in areas near where Kurdish fighters operate along the border with Turkey, from the Euphrates River east toward Iraq. U.S. officials hope those Arab fighters, along with Kurdish units, can help isolate Raqqa and the Islamic State leadership there.

Even though Kurdish forces are more numerous in that area and are seen by Washington as more effective, the Pentagon will not arm Syrian Kurds directly because of opposition from NATO ally Turkey, which fears strengthening Kurdish groups near its border.

The plan could eventually include cooperation with Arab fighters west of the Euphrates, around the town of Manbij, where the Islamic State is now advancing into the chaos of the ­anti-Assad fight.

Significant military aid to those fighters, in an area where Islamist extremist groups are mixed with and often fighting beside moderate opposition rebels, would mark a departure from previous U.S. policy. A senior administration official, who spoke on the condition of anonymity to discuss the matter, declined to give specifics on any new aid that might arrive in northwest Syria. But the official said that “these supplies will be delivered to anti-ISIL forces whose leaders were appropriately vetted,” and described them as “groups with diverse membership.” ISIL is an acronym for the Islamic State.

The Pentagon believes U.S. support could help military forces seal Syria’s border and choke off the flow of fighters and weapons going to the Islamic State, although it might also put U.S. aircraft in close proximity to the Russian planes now operating on Assad’s behalf in western Syria.

Brett H. McGurk, a senior State Department official, said the changes reflected the Obama administration’s adaptive approach to developments within Syria. “The more we try to rigidly fit a square peg into a round hole . . . the less effective we’ll be,” he told reporters.

While the units receiving support under the Pentagon’s new plan are supposed to be primarily battling the Islamic State, it appeared likely that Assad and the Russians would still view them as a target.

The new program is separate from a CIA-led effort to aid rebel factions in Syria. It was not immediately clear how Friday’s announcement might affect the CIA program.

It also was unclear whether the Obama administration has committed to protecting the rebel units it will support if they are attacked by Syrian or Russian forces. After fighters trained by the United States were attacked this summer by a rival group, the United States launched airstrikes to support them.

Whitlock reported from London. Liz Sly in Beirut and Brian Murphy and Dan Lamothe in Washington contributed to this report.