A coalition of more than 70 partners, including the United States, has pledged to send millions of dollars and communications equipment to Syria's opposition groups.
The move signals deeper involvement in the conflict amid a growing belief that diplomacy and sanctions alone cannot end the Damascus regime's repression.
The shift by the US and its Western and Arab allies toward seeking to sway the military balance in Syria carries regional risks because the crisis there increasingly resembles a proxy conflict that could exacerbate sectarian tensions. The Syrian rebels are overmatched by heavily armed regime forces.
The summit meeting of the "Friends of the Syrian People" follows a year of failed diplomacy that seems close to running its course with a troubled peace plan led by UN-Arab League envoy Kofi Annan.
Indeed, US secretary of state Hillary Clinton and other participants at the conference in Istanbul uniformly expressed concern that Mr Annan's plan might backfire, speculating that Syrian president Bashar Assad would try to manipulate it to prolong his hold on power.
Mrs Clinton said she was waiting for Annan's report to the UN Security Council on Monday on the status of his peace plan.
She said: "There cannot be process for the sake of process. There has to be a timeline. If Assad continues as he has, to fail to end the violence, to institute a ceasefire, to withdraw his troops from the areas he has been battering ... then it's unlikely he is going to ever agree.
"Because it is a clear signal that he wants to wait to see if he has totally suppressed the opposition. I think he would be mistaken to believe that. My reading is that the opposition is gaining in intensity, not losing."
Mrs Clinton said the United States is providing communications equipment to help anti-government activists in Syria organise, remain in contact with the outside world and evade regime attacks.
The Syrian regime agreed last week to Mr Annan's plan, which calls for an immediate ceasefire, humanitarian access to besieged civilians and a political negotiation process led by Syrians. Since then, there have been daily reports of violence, including shelling on Sunday in the central city of Homs that activists said killed more than two dozen people.