Mitt Romney led fresh Republican attacks on Friday over Vice President Joe Biden's debate remark about security concerns at the U.S. diplomatic compound in Libya where four Americans - including Ambassador Christopher Stevens -- were killed in a terrorist attack.

The GOP presidential challenger accused Biden of contradicting State Department officials when he said at Thursday's debate against Romney's running mate, Rep. Paul Ryan, that "we weren't told they wanted more security" at the diplomatic facility in Benghazi.

The mission came under attack by armed militants on the 11th anniversary of the September 2001 al Qaeda attacks on New York and Washington.

However, State Department officials told a congressional hearing on Wednesday that they requested more security but were turned down by headquarters in Washington.

"When the vice president directly contradicts the sworn testimony of state department officials ... the American people deserve to know what happened," Romney told a campaign event in the battleground state of Virginia.

Biden made no mention of Libya in his lone campaign appearance on Friday in Wisconsin, instead focusing on his main campaign themes that Romney and Republicans favor protecting the wealthy over helping the middle class.

The vice president's comment in the feisty debate with Ryan provided an opening for Romney's campaign to continue its criticism of the Obama administration's foreign policy, a topic on which polls show voters favor the president over his Republican opponent ahead of the November 6 election.

Ryan referred three times during the debate to television images from Libya and elsewhere in the Arab world in recent weeks that he said showed an unraveling of President Barack Obama's foreign policy.

At the White House, spokesman Jay Carney responded Friday by saying Biden's remarks on Libya correctly reflected that any requests for diplomatic security are handled by the State Department. According to Carney, Obama and Biden were unaware of the specific requests mentioned at the House Oversight Committee hearing.

Biden "certainly was aware" of the congressional testimony when he responded to questions at the debate, which showed that the vice president was referring to himself and Obama in saying that they didn't know about the request for more security, Carney said.

The White House spokesman also repeated earlier assertions that there was no "actual intelligence" beforehand indicating that a terrorist attack on the Benghazi compound was imminent, and noted a formal State Department review was underway.

Secretary of State Hillary Clinton, in a speech on democratic changes in northern Africa, promised the United States would work to bring the Benghazi attackers to justice and that the review she ordered would get to the bottom of what happened.

At the same time, Clinton noted no one expected democratic change in the Arab world to happen quickly or easily. She said the United States must keep up its work promoting democracy, even if that puts diplomats in dangerous and volatile places.

"We will never prevent every act of violence or terrorism or achieve perfect security," Clinton said in the speech at the Center for Strategic and International Studies. "Our people cannot live in bunkers and do their jobs. But it is our solemn responsibility to constantly improve, to reduce the risks our people face and make sure they have the resources they need to do those jobs we expect from them."

At Thursday's debate, Ryan focused on the Libya incident.

The conservative House Budget Chairman accused the administration of failing to properly secure the compound, then giving an incorrect initial explanation for what happened by describing the attack as an anti-American protest, instead of a planned terrorist assault.

Biden smiled and shook his head as Ryan delivered his criticism, then responded that "not a single thing he said was accurate."

However, Biden then denied any knowledge of requests to increase security at the Benghazi compound, saying: "We weren't told they wanted more security there. We did not know they wanted more security."

The Romney campaign blasted Biden for the remark after the debate, releasing a statement that listed a string of media reports pointing out Biden's apparent mistake.

Democratic Rep. Chris Van Hollen of Maryland told CNN on Friday that any information about requests for more security in Benghazi never reached the White House.

"What the vice president is saying is that he and the president didn't know," said Van Hollen, who helped Biden prepare for the debate. "This information had been communicated, at least according to the hearings, to the diplomatic security folks at the State Department and some others. But it wasn't communicated to the president."

Biden headed for Wisconsin on Friday for a campaign event, while Romney and Ryan planned to team up later in the day for a rally in Ohio. With less than four weeks until Election Day, the candidates are focusing on the battleground states still considered up for grabs and vital to each side's chances for victory.

At their lone debate, Biden and Ryan challenged each other's facts and claims while offering starkly different visions for the direction the country should follow.

Biden achieved his goal of bringing the fight to his GOP foe in battling Ryan to a draw, boosting the spirits of Democrats disheartened by Obama's lackluster performance in the first presidential debate last week.

Ryan, meanwhile, made a positive impression with his command of both domestic and foreign policy issues that showed him to be a formidable national candidate after a career of local congressional races in his native Wisconsin.