Obama's fresh aid pledge comes days after the White House made clear it was upping military support for Syrian rebels battling President Bashar al-Assad's forces, including small arms and ammunition.
Previously, Washington had confined its support for Syria to non-lethal aid and humanitarian assistance.
Some 1.6 million Syrians have fled since the Syria conflict began in 2011, mostly to neighbouring nations.
Another 4.25 million have been displaced within the country itself, according to the United Nations.
Obama was due to announce the new US aid at a dinner of G8 leaders at the Lough Erne golf resort in Northern Ireland.