Mitt Romney on Wednesday hit out at the Obama administration's "disgraceful" response to violent protests in Egypt and Libya, accusing it of sympathising with the Islamist demonstrators.
"I'm outraged by the attacks on American diplomatic missions in Libya and Egypt and by the death of an American consulate worker in Benghazi," the Republican Presidential candidate said in a statement.
"It's disgraceful that the Obama Administration's first response was not to condemn attacks on our diplomatic missions, but to sympathize with those who waged the attacks."
The Obama campaign quickly fired back, with spokesman Ben LaBolt accusing Romney of launching a "political attack" on a day of tragedy.
"We are shocked that, at a time when the United States of America is confronting the tragic death of one of our diplomatic officers in Libya, Governor Romney would choose to launch a political attack," he said.
The US official was killed when an armed mob attacked the consulate in the eastern Libyan city of Benghazi hours after demonstrators scaled the walls of the US embassy in Cairo and tore down the US flag.
The demonstrations, which erupted on the 11th anniversary of the September 11 attacks, were motivated by outrage over a video deemed offensive to Islam that was posted on the Internet.
The US embassy in Cairo issued a statement earlier on Tuesday condemning "the continuing efforts by misguided individuals to hurt the religious feelings of Muslims, as we condemn efforts to offend believers of all religions."
Secretary of state Hillary Clinton issued a sharper statement later, saying there was no excuse for the violence.
"Some have sought to justify this vicious behavior as a response to inflammatory material posted on the Internet," Clinton said in her statement.
"The United States deplores any intentional effort to denigrate the religious beliefs of others. Our commitment to religious tolerance goes back to the very beginning of our nation.
"But let me be clear: There is never any justification for violent acts of this kind."
The Politico news website later quoted an Obama administration official as saying the earlier embassy statement "was not cleared by Washington and does not reflect the views of the United States government."
Romney and US President Barack Obama have stepped up attacks on one another since their party conventions last week and the week before, as the campaign enters a crucial final phase leading up to the November 6 vote.
Throughout the campaign, Romney has largely ceded the foreign policy debate to Obama, preferring to focus on the struggling US economy, where the president is seen as much weaker.
But the protests in Egypt and Libya appeared to provide an opening for Romney to renew an older line of attack in which he accused Obama of apologizing to the world for America's foreign policy.