The SpaceX founder has dispatched some 20,000 satellite units to maintain high-speed internet connections, allowing Ukrainian troops to operate drones, receive vital intelligence and communicate on the front line where there are no other secure networks.
Last week Musk claimed the operation, partly paid for by Washington and crowd-funding, had cost his firm "$80 million and will exceed $100 million by the end of the year".
But on Friday, he told social media users that the bill to keep the service running cost his company some $20 million (NZ$35m) per day.
"We are not in a position to further donate terminals to Ukraine, or fund the existing terminals for an indefinite period of time," SpaceX's director of government sales wrote, according to US broadcaster CNN.
Among the documents sent to the Pentagon was a direct request to Musk from General Valerie Zaluzhniy, the Ukrainian military's commanding general, for 8000 more Starlink terminals.
Nexta, an Eastern European news channel, said Musk was punishing "an entire nation" in his spat with the Ukrainian ambassador. Ukraine's ministry of defence shared a video of its troops praising the Starlink system.
Relations between Musk's firm and Ukraine have appeared to deteriorate significantly in recent weeks.
The billionaire denied last week reports he had spoken with Putin before publishing a "peace plan" on Twitter that heavily favoured Russian demands.
After Musk posted a poll on social media to gauge support for his plan, Andrij Melnyk, Ukraine's ambassador to Germany, told the SpaceX founder to "f*** off".
Responding to reports that Starlink was preparing to pull funding, Musk wrote on Twitter that he was simply following the ambassador's "recommendations".
In a separate report, it was claimed earlier this month Ukrainian troops had suffered outages of their Starling communication devices on the frontline, hindering efforts to liberate territory from Russian forces.
Starlink provides internet access using a mega-constellation of some 2000 satellites. To access the network, customers use special terminals, providing coverage in areas without conventional connections.