US Secretary of State John Kerry says the United States will double its non-lethal aid to opposition forces in Syria to $US250 million.
And he said that foreign backers had agreed to channel all future assistance through the rebels' Supreme Military Council.
Mr Kerry stopped short of a US pledge to supply weapons to insurgents fighting to overthrow Syrian President Bashar al-Assad that the rebels have sought.
Reuters reports he was speaking after a meeting of the Syrian opposition and its 11 main foreign supporters in Istanbul.
Mr Kerry urged other foreign backers to make similar pledges of assistance with the goal of reaching $1 billion in total international support.
A US official said earlier that new non-lethal US aid could include for the first time battlefield support equipment such as body armour and night-vision goggles.
The latest US expansion of non-lethal aid follows Mr Kerry's announcement in Rome in late February that Washington would shift policy to provide medical supplies and food directly to opposition fighters, an option it had previously rejected.
President Barack Obama has refused to supply arms to the rebels, reflecting concern that such weapons would fall into the hands of Islamist militants in the ranks of the fractious insurgency.