With the Syrian city of Aleppo facing a new regime onslaught and Washington’s diplomacy under criticism for failing to halt the carnage, the United States announced a new aid package.

The State Department said it would release a further USD 364 million to UN aid agencies and NGOs working to help vulnerable Syrian civilians inside and outside the war-torn country.

The latest slice of funding brings the total amount that the United States — the effort’s biggest single donor — has spent since the war started more than five years ago to USD 5.9 billion.

But the assistant secretary of state for population, refugees and migration Anne Richard confirmed it would come from funds already allocated by Congress and does not represent new US spending.

"And this is probably… our last announcement of the fiscal year," she told reporters at a briefing to announce the measure three days before the end of the 2016 government spending period.

The civil war in Syria erupted in early 2011, when dictator Bashar al-Assad unleashed a brutal clampdown on anti-government protests, and has since killed more than 300,000 people.

Millions have fled their homes, with many living in makeshift camps inside Syria or its neighbors Turkey, Jordan and Lebanon, reliant on UN agencies funded by major UN members.

The United States and Russia, which back the opposition and regime, respectively, announced a ceasefire on September 9. It lasted barely a week.

As the deal collapsed, Assad’s regime launched a major offensive against the rebel-held east of Syria’s commercial hub, Aleppo, backed by waves of Russian bombers.

The diplomatic initiative’s failure has exposed President Barack Obama’s White House to criticism that, having no back-up plan, it has effectively ceded the initiative to Russia.

White House spokesman Josh Earnest insisted yesterday that Obama "and his team are always looking carefully at the situation to determine if there is something different that we can do."

But the violence in Aleppo is such that air-dropping aid would be too risky, he said, while arming the rebels would "only further militarise a situation that doesn’t have a military solution."