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]