The president said the U.S. has "no intentions of attacking" North Korea but "reserves all options to defend our friends in the region."
Since the detonation Monday, Democratic critics have blamed Mr. Bush for abandoning one-on-one talks with North Koreans, reported CBS News White House correspondent Jim Axelrod. But Mr. Bush rejected criticism.
"It is the intransigence of the North Korean leader, Kim Jong Il, that led to the current situation," he said.
On Iraq, Mr. Bush acknowledged "tough times" in the war-torn country where sectarian violence has surged recently. But, he added, "It is in our interests that Iraq succeed."
Mr. Bush staunchly defended his Iraq policy, saying that he had adjusted tactics to reflect changing conditions on the ground.
He was asked about a recent comment by the Republican chairman of the Senate Armed Services Committee, Sen. John Warner, that Iraq was drifting "sideways" and that the U.S. should consider major changes if Baghdad doesn't get the violence under control within the next few months.
"I appreciate Sen. Warner from going over there and taking a look," said Mr. Bush. "I completely agree."
Still, he insisted, "We're constantly changing tactics."
Mr. Bush dismissed as "just not credible" a that contends nearly 655,000 Iraqis have died because of the war.
The study was based on interviews by researchers with Iraqi families and suggests a far higher death toll than other estimates.
Mr. Bush, who in the past has suggested 30,000 civilian deaths in Iraq, would not give a figure for overall fatalities.
"A lot of innocent people have lost their life," he said.
With just four weeks before the midterm elections, Mr. Bush acknowledged that the war in Iraq is having a political impact. It is "tough on the American psyche," he said, repeating a phrase he had used before.
Mr. Bush said there were "loud voices" in the Democratic Party for him to withdraw troops. But, he said, he was not going to "get out before the job is done."
"You empower your generals to make the decisions on what you do to win," he said.
If Mr. Bush's campaign message has been drowned out lately, his confidence remains high. Despite polls suggesting otherwise, he still says Republican will hold the House and Senate when voters cast their ballots, reports Axelrod. In the House, Democrats need a 15-seat pickup to gain control. In the Senate, they need six.
Mr. Bush said the leading issue for voters should not be the war, but the economy, where he pointed to signs of significant improvement in job creation, lower energy prices and tax cuts that he said are working.
Mr. Bush also reiterated his support for embattled House Speaker Dennis Hastert, who's faced criticism for his handling of the .
He said he doesn't think the scandal has undermined Hastert's credibility as a leader.
"Denny's very credible as far as I'm concerned," he said.
"I think the speaker's strong statements have made it clear to not only the party ... but to the country, that he wants to find out the facts," Mr. Bush said. "This is disgusting behavior when a member of Congress betrays the trust of the Congress and the family that sent a young page to serve."