Washington has been by far the single biggest humanitarian donor to Syrians and the additional funds will bring the sum it has spent since the start of the conflict in early 2011 to $4.5 billion.
But, by its own standards, it has been slow to accept Syrian refugees for resettlement, with barely 1,800 having arrived in the past four years -- a tiny fraction of the millions who have fled their homes.
President Barack Obama has promised that 10,000 will be admitted over the next fiscal year, but the main US priority is helping refugees in camps in the region while seeking a long-term settlement to the conflict.
"There are over four million Syrian refugees in the world today, the vast majority of whom receive support in the first country to which they flee," the State Department said.
"It is our hope that by increasing humanitarian assistance and protection efforts in Syria and neighboring countries, Syrians will not be forced to seek assistance abroad at greater personal peril, and will also be able to return home more easily when the conflict ends."
The extra funds will be shared between UN agencies, aid NGOs and regional governments and will be spent both inside Syria and in the camps in frontline countries.
The United States is leading a coalition of Western and Arab allies to carry out air strikes against the Islamic State jihadist group in eastern Syria and northern Iraq.
But its parallel efforts to isolate Syrian strongman Bashar al-Assad's regime while building up opposition forces and political groups and seeking a political settlement to the conflict have made little progress.