Diplomats and analysts said if Obama chooses not to arm the rebels, or to take a more active role in coordinating the flows of arms and money from others, he may find states such as Saudi Arabia and Qatar pouring in even more money and materiel.

The danger, they said, is that this could accelerate a trend in which outside powers arm and fund preferred militants in Syria, creating client militias beholden to their patrons and undercutting efforts to develop a unified rebel front.

“If there is no (U.S.) decision this week, I think other actors will act. The Arabs can’t afford to lose Syria,” said a diplomat who spoke on condition of anonymity.

“If we don’t act … you’re going to end up with Arab arms and Euro arms being provided,” said Aaron David Miller, a former senior State Department official now at the Woodrow Wilson Center for International Scholars think tank in Washington.