WASHINGTON — President Obama requested $500 million from Congress on Thursday to train and equip what the White House is calling “appropriately vetted” members of the Syrian opposition, reflecting increased worry about the spillover of the Syrian conflict into Iraq.

The training program would be a significant step for a president who has consistently resisted providing military aid to the rebels in the conflict in Syria, and has warned of the dangers of American intervention. But military and State Department officials indicated that there were not yet any specific programs to arm and train the rebels that the money would fund, nor could administration officials specify which moderate Syrian opposition members they intended to train and support, or where they would be trained.

“While we continue to believe that there is no military solution to this crisis and that the United States should not put American troops into combat in Syria, this request marks another step toward helping the Syrian people defend themselves against regime attacks,” Caitlin Hayden, a National Security Council spokeswoman, said in a statement. She also said the money and training would help Syrians fight extremists “who find safe haven in the chaos.”