Donald Trump's top general has been handed a startling secret report warning that China is selling weapons to America's Middle East allies as US forces burn through their stockpiles.

The analysis, drawn up this week by the Joint Staff's intelligence directorate for Dan Caine, chairman of the Joint Chiefs of Staff, has sparked alarm inside the Pentagon, and lands as Trump prepares to meet Xi Jinping in Beijing on Thursday.

With America depleting its arsenal to defend Gulf allies from the Iranian barrage, China stepped in to fill the shortfall, selling weapons to the very US allies Washington was scrambling to protect.

The intelligence analysis, reported by the Washington Post, did not name the Gulf states which China supplied, but Saudi Arabia and the United Arab Emirates have for years been Beijing's biggest regional customers.

China has also supplied energy to countries suffering from Iran's closure of the Strait of Hormuz, among them Australia, Thailand and the Philippines, all US treaty allies.

Trump's summit with Xi was postponed in March because of the Iran war, and he arrives in Beijing with his standing battered at home and abroad over the conflict which has wrought chaos on the global economy.

The President remains adamant that Iran cannot get its hands on a nuclear bomb, but is increasingly eager to end the war which has sent oil prices skyrocketing ahead of crucial midterm elections in November.

China has seized on the 39-day conflict to expand its military, economic and diplomatic influence around the world, the intelligence report warns.

Beijing has cast itself as the responsible global power, branding the US strikes on Iran 'illegal,' calling for restraint, and offering itself as a stabilizing alternative to Trump.

'The war in Iran is massively improving China's geopolitical position,' Jacob Stokes, a senior fellow at the Center for a New American Security, told the Washington Post.

Trump has claimed that the closure of the Strait of Hormuz is far more damaging to China than to the US, with roughly 45 percent of Beijing's crude oil imports passing through the narrow waterway.

But the intelligence warns that China has been able to weather the storm because it has vast oil reserves and has been building up its sources of renewable energy.

'China is the second-most-insulated country in the world to the energy crisis, after only the US,' Ryan Hass, a China expert at the Brookings Institution, told the Post.

China has been able to supply jet fuel and other energy commodities to other nations, even as Iran's blockade chokes off global flows.

'This is not altruism,' Hass added. 'It is Beijing seizing on an opportunity to drive wedges between America and its traditional partners.'

The US has raced through its missile stockpiles in the asymmetric fight against Iran's vast fleet of drones, firing million-dollar interceptors at cheap one-way attack aircraft.

US forces have burned through nearly half their entire Patriot interceptor stockpile, more than half their THAAD ballistic missile defenses and over 850 Tomahawk cruise missiles during the 39-day war, according to a Center for Strategic and International Studies analysis. Replacing them will take three to five years.

The drain has sparked alarm in Taiwan, Japan and South Korea, where officials are questioning whether the US can be relied upon to defend them against China.

Pentagon spokesman Sean Parnell said: 'Assertions claiming the global balance of power have shifted towards any nation other than the United States of America are fundamentally false.'

He added that the US 'maintains a deep, resilient arsenal and the industrial capacity required to defend our nation, protect our interests, and deter any adversary.'

White House spokeswoman Olivia Wales said the US 'decimated the Iranian regime's military capabilities in 38 short days and is now strangling what's left of their economy with one of the most successful naval blockades in history.'

Trump meets Xi in Beijing on Thursday, the first face-to-face between the two leaders since the Iran war erupted on February 28.

The President is expected to press Xi to lean on Tehran to accept a ceasefire and reopen the Strait of Hormuz, while Xi is widely expected to push for tariff relief and access to American-designed AI chips.

The summit was originally slated for March but was postponed when the war broke out.