Brussels, Belgium ? The United States and allied nations must not wait for “absolute proof” of an impending terrorist attack before acting to stop it, Defense Secretary Donald H. Rumsfeld said Thursday.

Addressing a meeting of defense ministers from the 19 NATO nations, Rumsfeld said the world has consistently underestimated the threat of terrorist attacks. The only surprise, he said, is that “we are still surprised when this happens,” according to an outline of the main points he made. The outline was provided to reporters; his speech was not open to coverage.

Rumsfeld said it is inevitable that terrorists will gain access to nuclear, chemical or biological weapons and that governments as well as the general public need to better understand the threat.

“Real situation worse than what facts show,” was among the key points in the outline of his remarks.

Another point: “Absolute proof cannot be precondition for action.” Although the outline of his remarks made no mention of Iraq, the point about acting in a timely way fits with Rumsfeld’s expressed view that in some cases the United States must take pre-emptive action to protect America.

European nations have been skeptical of pre-emptive military action.

Earlier Thursday, another U.S. official, who spoke to reporters on condition of anonymity, said NATO defense ministers agreed to put new emphasis on improving their ability to detect, prevent and respond to attacks by weapons of mass destruction _ particularly biological, nuclear and radiological.

The agreement was part of a broader discussion, led by Rumsfeld, of ways the North Atlantic Treaty Organization can modernize its command structure and focus member countries’ spending on high-priority defense items as the alliance prepares to add new members.

The U.S. official said the ministers discussed a concern that is at the top of Rumsfeld’s list of priorities _ the nexus between terrorism and nations like Iraq that could provide terrorists with weapons of mass destruction. Other countries mentioned in this context were Iran, North Korea, Syria, Libya and Cuba, the official said.

There was no discussion of whether to undertake a pre-emptive attack on Iraq or other countries, the official said.

On Wednesday, Rumsfeld and his British counterpart singled out Iraq as a growing threat to the West.

“We know that Saddam Hussein’s regime in Iraq has had a sizable appetite for weapons of mass destruction,” and it is finding ways to acquire their ingredients, Rumsfeld said Wednesday.

“We know the borders into that country are quite porous,” he added, allowing Iraq to import technologies useful for both civilian and military industries “as well as illicit materials that are helpful in their programs for weapons of mass destruction.”

“There is not a doubt in the world that with every month that goes by, their programs mature,” he said in London before flying to Brussels for meetings Thursday and Friday with NATO allies.

Iraq denies it possesses or is developing weapons of mass destruction, but it has refused to allow the international inspections that it accepted as a condition of ending the 1991 Gulf War.

Rumsfeld would not discuss the possibility of U.S. military action to topple Saddam, saying that was a matter for President Bush to decide. He spoke at a joint news conference with British Defense Secretary Geoff Hoon after meetings to discuss Iraq and other issues.

The two defense chiefs flew together to Brussels, where Iraq is expected to be a topic of discussion in NATO meetings Thursday, including the first-ever session of the NATO-Russia Council.

After morning NATO meetings, Rumsfeld held a one-on-one session with his Russian counterpart, Sergei Ivanov. It originally was to last 40 minutes, to be followed by a joint news conference. But the session with reporters was scratched, and aides said the two officials planned to hold a second session Thursday evening after NATO’s afternoon conference was over. Aides offered no word on what Rumsfeld and Ivanov talked about or why a second session was needed.

Rumsfeld and Hoon both expressed their governments’ hope for a lowering of tensions between nuclear rivals India and Pakistan. Rumsfeld’s stop in London was the first on a 10-day journey that is scheduled to take him to the Indian and Pakistani capitals next week.

For months the Bush administration has been publicly making the case for strong action against Iraq, but allied nations have been slow to offer support.

Hoon said the Iraqi military threat has increased in recent weeks. Asked in a later interview to elaborate, Hoon said Iraq’s air defenses are more aggressively trying to shoot down the U.S. and British pilots who regularly fly combat air patrols over northern and southern Iraq.

He was alluding to the fact that U.S. and British pilots have reported a series of attacks in recent week by Iraqi anti-aircraft artillery and surface-to-air missiles. The allied planes have responded by bombing various elements of Iraq’s integrated air defense system.

Since the start of U.S. and British enforcement of the “no fly” zones more than a decade ago, Iraq has considered them a violation of its sovereignty and has vowed to shoot down pilots.