US Secretary of State Hillary Clinton answered Republican criticism of the Obama administration's handling of last month's attack at the US consulate in Benghazi, Libya, saying she - not the White House - is responsible for security at all of America's diplomatic missions.

"I take responsibility," Clinton told CNN. "I'm in charge of the State Department's 60,000-plus people all over the world (at) 275 posts."

With only weeks before the presidential election, outrage has crystallized around Vice-President Joe Biden's claim in last week's debate with Republican vice-presidential nominee Paul Ryan that "we weren't told" about requests for extra security at the consulate where assailants killed Chris Stevens, US ambassador, and three other US citizens.