US President Barack Obama will tell G8 leaders that Washington is offering a new $300 million aid package for refugees inside and outside Syria, a senior official said.
The new donation will bring the total of US humanitarian aid to Syrian refugees over $800 million (600 million euros) and will include food, water and materials for shelter, deputy national security advisor Ben Rhodes told reporters.
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.