Sanders apologizes to Clinton — and his supporters — for data breach during DNC debate
Sen. Bernie Sanders (I-VT) speaks during a Democratic presidential debate on Dec. 19, 2015. (ABC News)

U.S. Democratic presidential candidate Bernie Sanders apologized to front-runner Hillary Clinton at a debate on Saturday for a breach of Clinton voter files by a Sanders staffer that intensified tensions between the two camps.

"Yes, I apologize," Sanders said when asked about the controversy by ABC moderators at the debate, but he renewed his criticism of the Democratic National Committee for freezing access to his own voter files until the issue was resolved late on Friday.

Clinton, whose campaign said Sanders made a number of breaches into Clinton computer files, accepted the apology and said it was time to move on.

"I very much appreciate that comment, Bernie," she said. "Now that I think we’ve resolved your data, we’ve agreed on an independent inquiry, we should move on. I don’t think the American people are all that interested in this."

The two candidates, and a third, former Maryland governor Martin O'Malley, also debated how to take on Islamic State militants and protect Americans from lone wolf attacks like the Dec. 2 killings of 14 people in a shooting spree in San Bernardino, California.

Sanders noted he had voted against the 2003 U.S.-led war in Iraq and said he did not believe in unilateral American military action. Clinton, as a U.S. senator from New York, had voted to authorize the war in a vote she has since disavowed.

Clinton criticized Republican presidential front-runner Donald Trump for his proposal to ban Muslims from entering the United States. She said "the first line of defense against radicalization is in the Muslim community," and that all Americans should work with them.

The rising tensions between Clinton and Sanders, who have largely refrained from attacking each other, occurred at a crucial moment for Sanders, who is trying to erase the front-runner's lead in the November 2016 Democratic White House race just six weeks before Iowa holds the first nominating contest.

On Friday, Sanders filed a lawsuit to force the DNC to restore access to his voter files, which it had blocked after the Sanders campaign improperly accessed files generated by the Clinton campaign. The DNC and Sanders reached a late-night deal restoring access for Sanders.

Sanders campaign manager Jeff Weaver accused the DNC of working to protect Clinton, pointing to the party's limited debates at low-viewership periods such as Saturday nights as an example. Clinton campaign manager Robby Mook accused the Sanders campaign of stealing parts of its "strategic road map" for voter turnout in the primary battle.

The Sanders campaign said access to the files was restored early on Saturday. While the dispute may not mean much to voters, the bitterness could linger between the two campaigns.

The debate is the party's first since the attacks in San Bernardino, and Sanders also is eager to explore his differences with the former secretary of state on issues such as her support for a no-fly zone in Syria and her 2011 advocacy of regime change in Libya, his campaign said.

(Editing by Steve Orlofsky and Paul Simao)

Watch the exchange, as posted online, below.