WASHINGTON - US Vice President Joe Biden and his Republican challenger Paul Ryan locked horns over the deadly attack on the US consulate in Libya's Benghazi immediately after their debate began.

Picking up the Romney campaign's usual tone, Ryan slammed the Obama administration's weakness on foreign policy, saying it took two weeks for President Barack Obama to declare the Libya incident as a terror attack.

Biden criticized the Romney-Ryan ticket for making political statements even before they got to know more facts about the incident, which left the US ambassador and three others killed.

Meanwhile, Biden stressed that any mistakes made in response to the attack will not be repeated.

"I can make absolutely two commitments to you and all the American people tonight," Biden told the audience at Centre College in Danville, Kentucky.

"One, we will find and bring to justice the men who did this. And secondly, we will get to the bottom of it, and wherever the facts lead us, wherever they lead us, we will make clear to the American public, because whatever mistakes were made will not be made again," he said.

The Obama administration initially described the Benghazi attack on the night of September 11 as a spontaneous act ensuing from protests over a US-made film that denigrated the Prophet Mohammed, but it later called the assault a "terrorist act."

US Vice President Joe Biden (L) and Republican vice presidential nominee Paul Ryan react at the conclusion of the US vice presidential debate in Danville, Kentucky October 11, 2012. [Photo/Agencies]

The Republicans have raised sharp questions over the administration's handling of the security and protection of US facilities and personnel overseas.

Thursday's vice presidential debate will not decide the outcome of the November 6 showdown, but still matters in this close election, said Brookings Institute fellow Elisabeth Jacobs on Wednesday during a web chat.

"Debates can change the polls, but typically not in a fundamental way that decides the election," said Jacobs.

She expected Biden and Ryan to try to bring up issues which they have strong history in. Biden is expected to bring up foreign policy and women issues while Ryan is expect hammer the deficit issue.

The 69-year-old vice president, a former chairman of the Senate Foreign Relations and Judiciary Committees, is expected to highlight the Romney-Ryan ticket's lack of international experience during the debate.

The 42-year-old Ryan, a seven-term congressman and chairman of the House of Representatives Budget Committee, is a leading figure outlining the Republican party's budget plan.

Vice presidential debates traditionally don't carry the same influence as presidential debates in elections. But stakes have been raised for both campaigns over the 90-minute debate on Thursday evening.

After Obama's lackluster performance in the first presidential debate with Republican nominee Mitt Romney last week, Biden is under pressure to do well in this debate.

For Romney's side, another win could help the Romney-Ryan ticket hold the momentum heading into the November showdown.

Ahead of the debate, campaign spokeswoman Jen Psaki told reporters on Thursday that Biden "is pretty fired up and ready to go this evening for the debate."

The latest Pew survey released on Wednesday found that voters are divided over who will do better in Thursday's vice presidential debate, with 40 percent expecting Ryan to be the winner and 34 percent favoring Biden.

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