President Bush said Thursday that the United States will not be bullied by this week's killings of 25 service members in Iraq or what he called the "dark, dim, backwards" views propagated by al Qaeda's number two commander, Ayman Zawahiri, and other terrorists.

Speaking shortly after Zawahiri broadcast a new taped warning that the United States and its allies would suffer thousands of deaths if it did not pull out of Iraq, Bush struck a defiant tone, saying the United States will stay on the offensive to "complete the job in Iraq." He spoke dismissively of Zawahiri, his followers and their ideology and said the twin U.S. strategies to defeat terrorists inside and outside Iraq are working.

"We owe it to the American people, and other freedom-loving countries, to bring these killers to justice," Bush said in a news conference at his ranch here, where he met with Colombian President Alvaro Uribe. Terrorism was at the top of their agenda.

Bush, who hoped to turn attention to his domestic agenda this month, once again found himself consoling families, defending the war and insisting the United States will not reduce troop levels until it is clear the Iraqi army is strong enough to fend off the insurgents.

Some administration officials have been urging Bush to adopt a new language on terrorism, putting less emphasis on terms like "war" and more on the idea that the United States and allies are engaged in a long-term ideological struggle to push the Islamic world toward a peaceful and democratic future. On Thursday, however, Bush did not temper his words as he denounced "killers" intent on imposing "their dark vision on the world."

For the second straight day, Bush paid tribute to the 21 U.S. Marines from an Ohio regiment who were killed this week in combat, 14 of whom were blown up by a powerful roadside bomb on Wednesday. The people of Brook Park, where many of the soldiers were from, "have suffered mightily over the last couple of days," he said. The overall U.S. death toll topped 1,800 this week. "We're laying the foundation for peace for generations to come," Bush said. A new Associated Press-Ipsos poll shows approval for Bush's handling of Iraq at its lowest level yet -- 38 percent.

One week after a top U.S. commander suggested U.S. troops might begin to pull out next year if sufficient progress is made toward creating a new government and Iraqi military, Bush said the drawdown will begin "as soon as possible" but did not affix a timetable for such action.

In the short news conference, Bush also said he was confident that NASA officials would make wise decisions in bringing the Discovery space shuttle mission to a safe and successful conclusion. It's "important for the American people to understand, that, one, exploration is important; two, there will be some good coming out of exploration; and, three, that we've got a new vision embraced by NASA and its pioneers," he said. The rest of Bush's day was spent on his ranch, where he played host to Uribe for private talks and a family lunch.

Uribe faces his own terrorist threat at home, one that is jeopardizing the stability of both Colombia's government and its economic markets. He came to Crawford looking for a political lift and continued assistance from the United States. "The great enemy of Colombian democracy is terrorism," Uribe said.

Uribe's government is at war with rebels, funded largely by the drug trade, who have kidnapped and killed thousands of people in an attempt to impose a Marxist-style ideology in Colombia. In their private meeting, Bush said, they talked about terrorism, trade and human rights. Like U.S. relations with many nations involved in the war on terror, the relationship with Colombia is complicated by charges that the Uribe government encourages -- or turns a blind eye to -- abuses against its people and the spirit of democracy. "This was discussed with great seriousness and with great respect," Uribe said.

In the days leading up to Bush and Uribe's meeting, Amnesty International and other human rights groups challenged Bush to demand Uribe address questions about government collusion with paramilitary groups and provide impartial investigations of human rights violations.

The State Department announced this week that Colombia has met the human right standards required to receive U.S. financial assistance. The United States has provided more than $3 billion in assistance to Colombia over the past five years to help the government clamp down on cocaine and heroin production and defeat the leftist insurgency. About 25 percent of U.S. aid is conditioned on meeting human rights standards.

Of his talks with Uribe, Bush said: "I listened intensely and believe that he is interested in following through . . . so that the world will hear loud and clear that Colombia is a nation of law and human rights and human dignity."

Human rights advocates were distressed, criticizing Bush for being too credulous toward his visitor. "President Bush betrays his stated commitment to uphold human rights when he accepts President Uribe's rhetoric at face value," William F. Schulz, executive director of Amnesty International USA, said in a statement.

President Bush speaks at a news conference with Colombian President Alvaro Uribe after the two leaders met at Bush's ranch in Crawford, Tex.