WASHINGTON—A proposal to arm Syrian rebels was backed by the Pentagon, the State Department and the Central Intelligence Agency, but the White House decided not to act on the plan.

Defense Secretary Leon Panetta and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, revealed publicly for the first time at a Senate hearing on Thursday that they supported the proposal last year by senior officials including then-Secretary of State Hillary Clinton and then-CIA director David Petraeus.

The officials came to favor the plan last year with the meltdown of an international diplomatic initiative to end the Syrian civil war, according to current and former officials involved in the deliberations.

The White House stalled the proposal because of lingering questions about which rebels could be trusted with the arms, whether the transfers would make a difference in the campaign to remove Syrian leader Bashar al-Assad and whether the weapons would add to the suffering, the U.S. officials said. A U.S. official cited the findings of a CIA team of analysts, which cast doubt on the impact of arming the rebels on the conflict.

The disclosures thrust a spotlight on the extent to which President Barack Obama charts his own course in the face of calls to action by members of his own team, and on the extent of his caution about entering a new conflict. The White House declined to comment on internal administration deliberations.