US military newspaper Stars and Stripes carried the report on Friday, saying the demand was made by the US chief executive during a phone call with the Saudi monarch last December.
The money, it said, would be spent towards preventing the empowerment of Syrian President Bashar al-Assad and Syria’s allies, Iran and Russia. Washington would further the goal as far as bringing about "a political settlement of the war that ultimately includes the exit of both Assad and Iran,” it added.
It cited hawkish US Senator Lindsey Graham as reminding Gen. Joseph Votel, head of US Central Command earlier in the week of "the negative long-term impact that a win for Iran, Russia and Assad would have on US allies such as Israel and Jordan.”
The Islamic Republic has been successfully lending military advisory support to the Syrian government in its counter-terrorism operations against multiple foreign-backed militants wreaking havoc in the Arab state.
Russia also started lending its airpower to Damascus’ operations in September 2015.
The two have also been mediating talks between the Syrian government and opposition groups towards finding a political solution to the crisis.
"By the end of the call, according to US officials, the president believed he had a deal,” the paper said.
It said Riyadh acknowledges that it partly shares Washington’s goals "particularly that of limiting Assad’s power and rolling back Iran’s influence.”
After Israel, Saudi Arabia is the US’s biggest regional ally. Trump visited the kingdom last May in his first foreign trip, signing $110 billion worth of arms deals with Riyadh.
However, over the past two years, Syria and its allies have been making great strides against the militants, reclaiming one turf after the other.
The paper said, "The increasing likelihood of an Assad victory in the civil war has left many US policymakers and lawmakers aghast and the US mission in Syria jumbled and confused.”
Last December, Russia’s military chief said militants, including those with the Daesh terror group, were receiving training at US bases in Syria, adding that the terrorists had been instructed to "destabilize” the Arab country.
On March 1, a top Russian security official said the US had set up around 20 military bases in areas controlled by Kurdish militants it supports in northern Syria.
In January, the US revealed that it sought to create a 30,000-strong force, which uses the Kurds as its backbone, near the Turkish border in Syria. This prompted Turkey to begin attacking the Kurds in the northern region of Afrin.