President Joe Biden said on Sunday he remains "optimistic" about finding an agreement with his Republican opponents to raise the US debt limit and avoid a default, which his administration warned would cause "catastrophic" consequences.

Congressional Republicans are demanding budget cuts in exchange for lifting the US borrowing limit, while the White House has insisted for months that the nation's credit should not be up for negotiation.

Alarm bells are meanwhile ringing over the possibility of a first-ever US default, with uncertainty over the actual date the government would stop being able to pay its bills.

The two sides have remained at an impasse despite weeks of warnings from government officials and bankers that a default could unleash drastic consequences, including a possible recession and likely global financial contagion.

Nonetheless, Biden said on Sunday he thinks he will eventually be able to reach a deal.

"I remain optimistic because I'm a congenital optimist, but I really think there's a desire on their part as well as ours to reach agreement. I think we'll be able to do it," Biden told reporters while out on a bike ride near his beach home in Delaware.

A much-anticipated new round of debt-ceiling talks between Biden and Republican leaders, including House Speaker Kevin McCarthy, was postponed from Friday.

Asked if the Tuesday meeting was still on, Biden said: "I think so."

Treasury Secretary Janet Yellen has warned a default could occur by June 1, while the nonpartisan Congressional Budget Office forecast on Friday the date of June 15.

"We shouldn't be here," Deputy Treasury Secretary Wally Adeyemo told CNN on Sunday.

"If Congress failed to raise the debt limit by the time of default, we would go into a recession and it'd be catastrophic," he warned.

"The United States of America has never defaulted on its debt – and we can't." (AFP)