US Secretary of State Hillary Clinton assumed responsibility over four deaths in last month's attack on the US diplomatic mission in Benghazi, Libya, which has become an issue in the hard-fought US presidential campaign.

"I take responsibility," Mrs Clinton said on a visit to Peru, adding that President Barack Obama and Vice President Joe Biden would not be responsible for specific security instructions for US diplomatic facilities.

Her comments followed criticism of the Obama administration over the Benghazi attack, which Republican presidential challenger Mitt Romney has sought to use to dent Mr Obama's foreign policy credibility before the November 6 election.

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Republicans have focused on the shifting explanations for the attack, which Mrs Clinton said were the result of "the fog of war".

Mr Romney has accused the administration of not providing adequate security and misrepresenting the nature of the attack, which resulted in the death of four Americans.