Mitt Romney exploited an assault on the US embassy in Cairo, followed by the killing of the US ambassador to Libya, to reject what he sees as President Obama's apology-led approach to the region which denigrates US values.

For Obama, Romney's swift bid to wring political gain from a foreign policy catastrophe before the facts were known was proof the Republican simply lacks the discretion and temperament to take his seat behind the Oval Office desk.

In a brazen attack, Romney accused the Obama administration of being more interested in sympathizing with Islamist perpetrators of the attack on the Cairo embassy than defending American values.

His prop was an embassy statement issued before the post was invaded, which officials later insisted was not cleared in Washington.

The release decried "continuing efforts by misguided individuals to hurt the religious feelings of Muslims" in a slap at a new US film deemed offensive to Islam which had prompted uproar.

Romney built on his view that Obama's approach is somehow un-American, suggesting the administration cared more about not offending foreigners than protecting US Constitutional rights to freedom of speech.

"It's never too early for the United States government to condemn acts on Americans and to defend our values," Romney said, in an apparent effort to narrow Obama's electoral advantage in foreign policy.

A new Transatlantic Trends poll published Wednesday found 54 percent of Americans approve of Obama's handling of foreign affairs, while 66% are happy with his record on fighting terrorism.

But most other Republicans were less adamant in their criticism of Obama, leaving the party nominee exposed and facing claims that he had jumped the gun, in his bid to make political hay before the election on November 6.

Former George W. Bush speechwriter David Frum wrote in the Daily Beast that Romney's bid to "score political points on the killing of American diplomats was a dismal business in every respect" and politically "graceless and stupid."

Romney was already on thin political ice after a gaffe-strewn foreign tour in July cast doubt on his readiness to lead.

Obama's first on-camera response to the crisis was an appearance in the White House Rose Garden, in which he somberly condemned the attack on the US consulate in Benghazi, projected presidential dignity, called on Americans to unite and promised action to protect diplomats in the Arab world.

He mourned slain US ambassador Chris Stevens and his three comrades and vowed justice.

But the president's unspoken political mission was to exploit and enhance perceptions that he is a steely global leader and to shield his own election prospects from immediate damage from any voter outrage at home.

But his political shot was not long coming.

"There's a broader lesson to be learned here: Governor Romney seems to have a tendency to shoot first and aim later," Obama said in a CBS News interview.

"As president, one of the things I've learned is you can't do that --it's important for you to make sure that the statements that you make are backed up by the facts and that you've thought through the ramifications before you make them."

Senator John Kerry eviscerated Romney for playing politics as Democrats pushed the line that Romney had failed a critical test of his capacity to serve as US commander-in-chief.

"It is exactly the wrong time to throw political punches. It is a time to restore calm and proceed wisely," Kerry said.

Anthony Cordesman, a respected analyst at the Center For Strategic and International Studies, cautioned against political "overreaction."

"It may be the duty of opposition candidate to criticize and challenge, but not at the cost of America's strategic interests, lasting relations with key nations in the Middle East, or somehow making this an issue that puts Christian against Muslim or the West against the Arab world," Cordesman said.

Julian Zelizer, professor of history at Princeton University, said such crises pose peril, but also opportunity for candidates.

"A crisis like this can play both ways. For Barack Obama, the risk is clear," Zelizer said, warning a poor response could undermine the president's claim to foreign policy primacy.

Such situations allow voters to assess how potential leaders respond to a fast-moving crisis abroad, he said.