Democrat Hillary Clinton called for an international probe of Benazir Bhutto's killing as presidential candidates in both parties sparred over foreign policy six days before Iowa begins a close nominating race.

As the campaign hit high gear, Democrats Barack Obama and John Edwards exchanged fire over their ability to fight special interests and Republican Mitt Romney unveiled ads targeting rivals Mike Huckabee and John McCain, drawing a counterattack from McCain.

Clinton, in a three-way battle with Obama and Edwards for the lead in Iowa, questioned the credibility of Pakistan President Pervez Musharraf and said opposition leader Bhutto's assassination put a harsh light on President George W Bush's approach in the region. Nuclear-armed Pakistan is a vital ally in the US war on terrorism.

'It is clear the Bush policy of giving Musharraf a blank check has failed,' the New York senator said in Story City in north-central Iowa. 'We need an international and independent investigation into the death of Benazir Bhutto.'

Polls show a close race in both parties in Iowa, which on Thursday opens the state-by-state battle to choose Republican and Democratic candidates for the November 2008 election to replace Bush.

Clinton, Obama and Edwards are essentially deadlocked at the top among Democrats, polls show, while Huckabee and Romney battle for the Republican lead in a state where a win can provide valuable momentum.

Bhutto's killing on Thursday prompted candidates to exercise their foreign policy expertise and, in the case of Clinton and Edwards, tout their experience.

The Clinton and Obama camps traded accusations of politicizing her death, while Romney and Huckabee were put on the defensive over their lack of foreign policy credentials. Several Democrats leveled criticism at Musharraf.

'We have poured billions of dollars in support to President Musharraf -- and he has not focused on dealing with the terrorist threat that is growing,' Obama, an Illinois senator, said in Willamsburg, Iowa, stopping short of calling for Musharraf's ouster.
Bill Richardson, governor of New Mexico and former US ambassador to the UN, criticised his Democratic rivals for not pushing for Musharraf's removal. He called for an end to US military aid to Pakistan not directly related to fighting terrorism until Musharraf resigns.

The debate over Bhutto's death came as nine Democratic and Republican candidates braved a fresh snowstorm to hit the road in Iowa and many launched a blizzard of new ads.

Romney, the former Massachusetts governor who has seen big leads disappear in Iowa and the state that holds the next contest, New Hampshire, launched direct attacks on his two nearest rivals in those states.

In New Hampshire, where John McCain has sliced his lead, he criticized the Arizona senator's stances on immigration and taxes in a new ad. 'John McCain, an honorable man. But is he the right Republican for the future?' asks the narrator.

The ad criticized McCain's backing for a Senate bill giving illegal immigrants a path to citizenship, a move that angered many conservatives, and his vote against two of Bush's early tax cut packages.

McCain fired back with his own harsh ad using quotes from some of the 20 newspaper endorsements he has earned in New Hampshire.

'The Concord Monitor writes 'If a candidate is a phony ... we'll know it.' Mitt Romney is such a candidate,' the ad's announcer says.

In Iowa, Romney released an ad attacking Huckabee, the former Arkansas governor who has rocketed into the lead in polls here.

The ad criticizes Huckabee's record in Arkansas of backing state benefits for illegal immigrants and granting more than 1,000 pardons and commutations. 'Two good men. But who is ready to make tough decisions?' the ad says.

Obama and Edwards, a former North Carolina senator, traded jabs over who would be best able to r