The admiral who runs America's submarine building program has confirmed construction is behind schedule and nowhere near the rate required to supply Australia's Aukus nuclear submarines on schedule.

R Adm Jon Rucker told the Naval Submarine League's annual symposium in Arlington, Virginia, last week that the US had "an exceptionally fragile" military shipbuilding base and could not meet construction rates for its own vessels this year.

In remarks reported by defence industry media, Rucker said a materials shortage had affected the sequencing order of manufacturing and slowed down the production rate. He appeared to cast doubt on his own timetable.

"I'll be frank - there are some risks to achieving these goals," Rucker was reported as saying. "We have done great things and we've made progress, but more is needed. This is our north star ... This is the challenge of our time."

The US Navy was meant to hit a production rate of two submarines a year in 2028 and must reach 2.33 to fulfil its commitment to deliver three Virginia-class nuclear-powered attack submarines to Australia in the 2030s, while also meeting its domestic commitments.

But Rucker, who is executive officer of the US attack submarine program, revealed it would not reach its most immediate goal of 1.5 by the end of 2024.

"We continue to hover around a production rate of 1.1 to 1.2 boats per year," Rucker was reported as telling the conference. "Our goal at the end of this calendar year was to be at 1.5. We had a threshold of 1.3. Right now, we're tracking the threshold value. We will not make the goal value. It will be closer to 1.3."

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Despite this, Rucker insisted the program could exceed its 2028 ambition and reach a production rate of three by then - two Virginia-class submarines plus one of the new, larger Columbia-class vessels - alongside its sustainment obligations to Australia.

Rucker said building its own planned Columbia class boats was its "number one priority".

Addressing the same conference, the US's director of Navy reactors, Adm Bill Houston, said the nature of global threats meant "there may be a need for more Columbias" - appearing to signal the production pressures may only increase.

"We are not in low-rate production," Houston was reported as saying. "We are in the highest rate of production we've been in as a nation with an industrial base that's less than half the size [than in the cold war]. It's an exceptionally fragile industrial base. It's with an industrial base that is very, very challenged."

Asked about the schedule during a news conference alongside Australia's defence minister, Richard Marles, after talks in Darwin on Sunday, the US defence secretary, Lloyd Austin, said he was "confident" the submarines would be provided.

"Now, we recognise that there are challenges in the industrial base and we're doing things to address those challenges," he said.

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Austin said he had met the leaders of the companies involved and was encouraged by "their focus to get this done, and they will get it done".

"I would point out that we've met every benchmark that we've set out for ourselves with Aukus to this point."

Asked to respond to Rucker's comments, a spokesperson for Marles pointed to his Sunday endorsement of Austin's remarks.

"We do have a sense of confidence about the fulfilment of the timelines that we articulated when we announced the optimal pathway back in March of last year," Marles told journalists on Sunday. "Is Aukus deeply embedded? I think the answer to that is 'yes'."

Marles suggested the incoming Trump administration would honour the Aukus submarine deal, pointing to its bipartisan support in the US Congress.

"We are seeing Aukus survive change of governments in Australia, in the United Kingdom, and it will happen in America as well.

The Greens' defence spokesperson - and Aukus critic - Senator David Shoebridge, said Rucker's comments proved the deal was "a mess".

"The head of the US Navy's Virginia submarine program has just dropped a truth bomb on Aukus," Shoebridge said.

"Even after Australia has agreed to gift them $5bn of Australian public money to expand their submarine industrial base, the US is barely producing half the number of submarines it needs. Unless the US has enough for its own needs, does anyone seriously think either President Trump or whoever follows him will selflessly agree to hand any submarines over to Australia?"

On Tuesday, the defence departments of the three Aukus partner nations announced they had signed an agreement to use each other's testing facilities for hypersonic vehicles and systems.