US House Speaker John Boehner (R-OH) addresses reporters during a news conference with fellow House Republicans at the US Capitol in Washington October 4, 2013. [Photo/Agencies]

WASHINGTON - Republican House Speaker John Boehner vowed on Sunday not to raise the US debt ceiling without a "serious conversation" about what is driving the debt, while Democrats said it was irresponsible and reckless to raise the possibility of a US default.

"The nation's credit is at risk because of the administration's refusal to sit down and have a conversation," Boehner told ABC's "This Week," adding that there were not enough votes in the House of Representatives to pass a "clean" debt limit bill, without any conditions attached.

Asked if that meant the United States was headed towards a default if President Barack Obama does not negotiate, Boehner said: "That's the path we're on."

The comments appeared to mark a hardening since late last week when Boehner was reported to have told Republicans privately that he would work to avoid default, even if it meant relying on the votes of Democrats, as he did in August 2011.

Republicans and Democrats also traded blame for a shutdown that has brought much of the government to a standstill for nearly a week. With no end in sight, the battle over funding the government looks like it will merge with the one over the debt ceiling, which must be raised by October 17 to avoid default.

Republicans are seeking concessions in exchange for raising the nation's $16.7 trillion debt limit. If the borrowing cap is not increased, the United States could go into default, with what officials and economists say would be seriously damaging consequences for the US and global economies.

Treasury Secretary Jack Lew warned of serious consequences "if we were to have the unthinkable happen, to have the United States default."

"It is irresponsible and it is reckless to take that chance, which is why Congress needs to act," he told Fox News.

Pressed on Obama's vow not to negotiate over the debt limit, Lew said: "The president has always been looking for a way to negotiate, to find that reasonable middle ground, with a bipartisan group of members and senators to do the right thing for the American people ... He put out a budget that actually took an enormous step to do that. So the president is open to negotiation. The question about the debt limit is different."

Senator Ted Cruz, who has been the standard-bearer for Republican opposition to funding the government without measures to undercut Obama's landmark 2010 healthcare reform, also known as Obamacare, told CNN that Congress frequently in the past had attached curbs on spending to votes to raise the debt ceiling.

On this occasion, he said, Republicans were looking for three things before raising the debt ceiling: a significant structural plan to reduce government spending, no new taxes, and measures to "mitigate the harm from Obamacare."

Democrats vow not to negotiate on the funding bill or the debt ceiling, arguing that it is the job of Congress to pay its bills.