WASHINGTON - Iraq fired at U.S. and British warplanes flying over southern Iraq yesterday, an action the Bush administration called a "material breach" of a new United Nations Security Council resolution intended to disarm Saddam Hussein's regime.

Iraq has often fired on aircraft patrolling "no-fly zones" in northern and southern Iraq. And the allies' response yesterday was no stronger than in many previous clashes: They retaliated by bombing an Iraqi radar site near An Najaf, about 85 miles southeast of Baghdad.

This time, though, the Iraqi action defied what the United States says are new rules that forbid Iraq to fire on planes patrolling the no-fly zones. The United States said such an attack is a "material breach," meaning it is serious enough to be considered by the Security Council and used in building a case for war.

But Bush administration officials indicated that no broader U.S. military action is planned. At the same time, by calling the Iraqi attack a "material breach," an official said, Washington has the option of referring it to the Security Council.

Yesterday's U.S. retaliation was the first strike on Iraq since Wednesday, when Hussein's regime accepted the Security Council resolution demanding that it disarm and allow unconditional weapons inspections.

The new rules are buried in an obscure - and hotly disputed - paragraph of the resolution that the council adopted unanimously last week.

The paragraph reflects the deep divisions between the United States and other council members that were papered over to allow the council to speak with one voice against Iraq. These differences are surfacing over what some countries see as American eagerness to find a pretext for war.

The paragraph bars Iraq from threats or hostile acts against personnel of "any member state taking action to uphold any Security Council resolution."

The United States and Britain say that the patrols of the no-fly zones are carried out under two previous U.N. resolutions and are intended to prevent Hussein's regime from attacking Kurds in northern Iraq and the Shiite population of the south.

"Iraq's failure to comply with its obligations under Paragraph 8 of the resolution would constitute a material breach," Julie Reside, a State Department spokeswoman, said last night.

Under the resolution, member countries or U.N. inspectors can report violations to the council. The Security Council would then meet to assess whether the violations amount to a "material breach," which is grounds for military action. The United States has said it reserves the right to launch military action on its own if the council fails to act quickly and forcefully.

The United States is virtually alone among Security Council members in arguing that the resolution applies to the no-fly zones. That interpretation came from the Pentagon and has been affirmed by the White House. Though Britain has not commented officially, some diplomats say London disagrees with Washington's view.

"This resolution is about effective disarmament of Iraq through inspections," a Western diplomat on the Security Council said yesterday. "It was not about no-fly zones."

The U.S. interpretation, the diplomat said, "would not be an understanding that would find sympathy, let alone consensus, in the council as a whole. It would be seen as an excuse to go to war and as a bad-faith interpretation of the resolution."

After the resolution was approved, the U.S. envoy to the United Nations, John D. Negroponte, assured representatives of other countries that it contained no "hidden triggers," or buried language that the United States would use as authority for war.

The first tests

Russia's ambassador to the United Nations, Sergei Lavrov, said the United States and Britain assured him before the resolution was adopted that the disputed paragraph bars Iraqi threats only against employees of U.N. inspections agencies or against others hired to help them. This would exclude pilots of U.S. and British warplanes.

The definition of what constitutes a "material breach" is also debated. The resolution says it includes "false statements or omissions in the declarations submitted by Iraq ... and failure by Iraq at any time to comply with, and cooperate fully in the implementation of, this resolution."

What this means in practice will be tested soon, with U.N. inspectors returning to Iraq on Monday. They are to begin inspections Nov. 27, earlier than expected. By Dec. 8, Iraq must make a complete disclosure of all its programs to develop weapons of mass destruction, a document that is supposed to be a guide toward eliminating those programs.

In the past, Iraq has sought to deceive U.N. inspectors and has issued declarations with false statements and gaps, inspectors say.

In advance of the inspections, the White House is taking a tough line to keep the pressure on Iraq.

"Our view is zero tolerance when it comes to complying and cooperating with this resolution," Scott McClellan, a White House spokesman, said Thursday.

U.S. officials want the inspectors to be aggressive and to focus at first on a few key sites to "expose Iraqi deception," as one U.S. official put it.

Other nations, and the inspectors, say one goal should guide them: to make the inspections as effective as possible.

Failure to cooperate

Omissions or errors in Iraq's Dec. 8 declaration would not constitute a "material breach," a Western diplomat said. They would have to be "coupled with a failure to cooperate with the resolution more generally."

As to what constitutes failure to cooperate, the general view on the Security Council is that the inspectors should make a report "if they feel the cooperation is not sufficient to allow them to do their job."

Mohamed ElBaradei, director general of the International Atomic Energy Agency, said in Washington this week that a minor unintentional omission by Iraq wouldn't be a material breach.

He pointed to the 1969 Vienna Convention on the Law of Treaties as a guideline. In that case, a material breach would be a "violation of a provision essential to the accomplishment of" the purpose of the resolution, which is disarmament.

At a briefing in New York yesterday, Hans Blix, executive chairman of the U.N. Monitoring, Verification and Inspection Commission, said weapons inspectors would be unlikely to complain to the Security Council unless they encountered a pattern of obstruction.

If the search teams and Iraqi escorts were delayed by one flat tire, he said, they might overlook it. "Four flat tires may be a different thing," he said.

A denial of access, Blix added, "could be a very significant matter."