Australia insists the AUKUS pact is on track despite US senators saying their submarine industrial base is at "breaking point" due to the extra demands.
Democratic Senator Jack Reed and Republican Senator James Inhofe wrote to US President Joe Biden on December 21, urging him to make sure America's submarine fleet didn't take a hit while propping up Australia's and the United Kingdom's.
The duo were both on the US Senate Armed Services Committee when the letter was written, although Mr Inhofe has since left the Senate.
Their letter suggests the AUKUS pact had become a "zero-sum game for scarce, highly-advanced US (nuclear submarines)".
"Over the past year, we have grown more concerned about the state of the US submarine industrial base as well as its ability to support ... (AUKUS)," the letter first reported by American website Breaking Defence reads.
"Current conditions require a sober assessment of the facts to avoid stressing the US submarine industrial base to the breaking point."
The senators went on to urge Mr Biden to adopt a "do no harm approach" to negotiations and ensure US national security capabilities wouldn't be hurt in the process.
Defence ministers from the three nations met last month, with Deputy Prime Minister Richard Marles saying afterwards he was pleased the US had indicated it would not leave Australia with a capability gap while it waits for the nuclear-powered submarines. They are not slated to be built and operating until 2040.
Australia remains on track to announce its optimal pathway to acquire the nuclear submarines in the first part of this year.
"As the Deputy Prime Minister has said, Australia is grateful for the work which we have been able to do with the United States and United Kingdom to enable Australia to acquire a nuclear-powered submarine capability, and the significance of the US working with the UK through the AUKUS framework to provide Australia with that important capability is not lost on us," a spokesperson for Mr Marles said.
"AUKUS will significantly transform Australia's strategic posture and the work undertaken over the last 16 months speaks to a shared mission between Australia, the United Kingdom and the United States."