Even as public support for his policy in Iraq continues to erode, President Bush (search) says the U.S. will "stay the course" and American troops will remain in the dangerous, war-ravaged country.

An AP-Ipsos poll taken early this week showed public support of Bush's handling of the war had dropped to 38 percent, the lowest since Bush ordered the U.S.-led invasion of Iraq.

The president, facing a grim and growing death toll, also said new threats of even more violence from Al Qaeda's second-in-command would not intimidate the United States into retreat.

At the same time Thursday, the U.S. military announced in Iraq that four more service members had been killed in action but also insisted American troops were making progress against insurgents.

"We will stay the course, we will complete the job in Iraq," Bush said in Texas as a videotape by Aymen al-Zawahiri, Al Qaeda's No.2, was broadcast around the world.

Al-Zawahiri threatened more destruction in London after subway and bus bombings and said in the videotape that the United States would suffer tens of thousands of military dead if it did not withdraw from Iraq.

Bush said that kind of talk only showed why the United States must remain in Iraq.

"They're terrorists and they're killers and they will kill innocent people ... so they can impose their dark vision on the world," Bush said as he stood alongside Colombian President Alvaro Uribe who was visiting at Bush's ranch in Texas.

Al-Zawahiri's threat was broadcast a day after the deadliest roadside bombing of U.S. troops in Iraq. The death of 14 Marines in that attack was a heavy loss to the 3rd Battalion, 25th Marines based in the Cleveland suburb of Brook Park. More than 1,820 members of the U.S. military have died since the beginning of the war in March 2003

Al-Zawahiri said the cause was "aggression against Muslims," and he threatened: "If you don't leave today, certainly you will leave tomorrow, but after tens of thousands of dead and double the number of disabled and wounded."

Bush said if terrorists think they can prevail in the Middle East, "they must not have understood the nature of our country. ... As I have told the American people, people like Zawahiri have an ideology that is dark, dim, backwards."

In Iraq, U.S. military spokesman Brig. Gen. Donald Alston defended operations in western Iraq, where there has been a rash of recent American deaths.

"We still have deaths. We still have suicide car bombs," he said. "But the numbers we see indicate (the insurgents) can't generate the same tempo, and I think that's because we've had some degree of effect in interdicting these forces."

Despite the deaths, he said the 13 car bombs reported in Iraq last week were the fewest in any week since April.

Still, there has been few obvious signs of progress in U.S.-led efforts to defeat the insurgency and to improve the Iraqi army and police so they can take over security responsibilities and allow the U.S. forces to leave.

The high recent death toll of American troops have dominated the news back home — no help for the already-eroding public support for Bush's Iraq policy.

The AP-Ipsos poll found that only 38 percent of Americans approve of Bush's handling of Iraq, the low point so far. A year ago, the public was evenly divided on Iraq, and confidence in Bush's stance on the war and terrorism helped him to election victory.