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Former presidential candidate John Edwards has endorsed Barack Obama

Barack Obama has rebuked Republican rival John McCain and President George W Bush for "dishonest and divisive" attacks in hinting that the Democratic presidential hopeful would appease terrorists, as a likely White House battle between the two candidates heated up even before the Democratic nomination is settled.

Obama, who holds what appears to be an insurmountable lead over Hillary Clinton for the Democratic nomination, responded forcefully to the criticism that started on Thursday, when the president first made the comments in the Israeli Knesset. Bush had ridiculed what he said were calls by some to negotiate with Iran and other US adversaries - remarks Obama's campaign interpreted as a jab at the candidate, who has advocated direct talks with Tehran.

While the White House denied Bush's comments were directed at Obama, they nonetheless sparked a war of words in which McCain weighed in.

The presumptive Republican nominee said Bush's warning raises the question of "why does Barack Obama... want to sit down with a state sponsor of terrorism." The Arizona senator, who has based much of his campaign on his national security and foreign policy experience, also charged that Obama was being naive.

Obama's response was swift.

"That's the kind of hypocrisy that we've been seeing in our foreign policy, the kind of fear-peddling, fear mongering that has prevented us from actually making us safer," he told a town meeting in Watertown, South Dakota.

Obama said McCain had a "naive and irresponsible belief that tough talk from Washington will somehow cause Iran to give up it's nuclear program and support for terrorism."

Bush' comments on Thursday, made while he was on a trip in part to mark Israel's 60th birthday, appeared to be an unusual foray by the president into the political fight. By tradition, partisan politics comes to a halt when a US president is on foreign soil.

Republicans increasingly see Obama, a first-term Illinois senator, as the all-but-certain victor in the fierce race for the Democratic nomination. He holds an ever-widening 1,900 to 1,718 delegate lead over Clinton, with 2,026 needed to clinch the nomination at the party convention in August.

Similarly, while he says has not taken for granted that he will win the party's nod, he has campaigned like the inevitable nominee - mindful of not alienating Clinton or her allies but also focusing his attention on McCain and linking him to the unpopular Bush. Other Democrats have followed suit.