The announcement means the US has now given $5.9 billion in assistance to the Arab country since the conflict started in 2011. The money will help the UN and independent charities offer food, shelter, safe drinking water and medical care. Millions in Syria need relief. And almost 5 million have fled as refugees to nearby countries.

"Getting help to people who desperately need it requires access," Anne Richard, the State Department's refugee chief, told reporters.

She criticised Syrian President Bashar Assad's government and its allies, Russia and Iran, for "bombing humanitarian convoys, hospitals and first responders trying desperately to keep people alive."

Syrian government forces have advanced in Aleppo, the country's largest city, since the latest US-Russian engineered cease-fire collapsed last week. The military has vowed to wipe out insurgents and more than 200 people have been killed in the latest offensive.

The US had contributed limited military assistance to some rebel groups fighting Assad, but has focused in recent months on securing a long-term truce.

Last week, the US accused Russia of bombing a humanitarian convoy on its way into Aleppo. Twenty civilians were killed. Russia denied responsibility.