WASHINGTON — The Obama administration on Friday abandoned its efforts to build up a new rebel force inside Syria to combat the Islamic State, acknowledging the failure of its $500 million campaign to train thousands of fighters and announcing that it will instead use the money to provide ammunition and some weapons for groups already engaged in the battle.

Defense Department training sites across the Middle East, including ones in Turkey and Jordan, will soon suspend almost all operations, officials said, in favor of a revamped program that briefly screens Arab rebel commanders of existing Syrian units before equipping them with much-needed ammunition and, potentially, small arms. Initial airdrops of equipment could begin as early as this weekend, officials said.

The decision to scuttle a central piece of President Obama’s strategy for confronting extremists in Syria was made after mounting evidence that the training mission had resulted in no more than a handful of American-coached fighters. And it comes amid Russia’s forceful entry into the Syrian conflict, a move by President Vladimir V. Putin that has highlighted the lack of progress by the United States and its coalition.

Senior officials at the White House and the Pentagon admitted that the strategy to pull fighters out of Syria, teach them advanced combat skills and return them to face the Islamic State had simply not worked, in part because many of the rebel groups were more focused on fighting the Syrian president, Bashar al-Assad.