In recent months, arming the rebels has gained support in the Senate from key Democrats and Republicans. Sen. Bob Corker of Tennessee, the ranking Republican on the Senate Foreign Relations Committee, urged Mr. Obama in a letter Monday to provide lethal aid to vetted elements of the Syrian opposition.

Doing so, Sen. Corker argued, will “shift momentum away from radical Islamist groups, the Assad regime and its militias toward more moderate elements and could help alter the balance of power on the ground at a time when negotiations over a political settlement have stalled.”

But two senior American officials said the proposal would mostly have symbolic value at this point because such arms are already widely available inside Syria and won’t make a real difference in the military balance in the country. Saudi Arabia, Qatar and Turkey are among the states that have been funneling arms, cash and other aid to groups of anti-Assad fighters that they support…

Though U.S. intelligence agencies generally agree that Mr. Assad has recently shored up his position, divisions remain there over his long-term prospects. Some intelligence agencies increasingly think Mr. Assad has not only turned the tide but could prevail in the conflict “sooner rather than later,” according to a senior U.S. official.

Other agencies are more cautious about his prospects. “It isn’t unanimous that Assad will stay in power,” another senior U.S. official said.