The latest threat was aimed at two of Washington's steadiest allies in Iraq: Poland and Bulgaria. It came from a previously unknown group calling itself the al-Qaida organization in Europe and it appeared on a Web site not normally used by Islamic militants.

The message warned both countries that, if they do not pull their troops out of Iraq, they will face bloodshed similar to that caused by the terrorist attacks in Madrid last March.

Security expert John Tesh, at London's Royal Institute of International Affairs, says such warnings have become commonplace, but he says they should not be ignored.

"I do not put particularly that much credibility on the local groups themselves, but to the extent that they're reflecting what seems to be a general policy of attempting to intimidate coalition partners or anyone really who's attempting to do anything in Iraq, then you have to believe that something might happen as a result," he said.

In Warsaw, Piotr Stasinski, deputy editor of the prestigious Gazeta Wyborcza newspaper, says Poland is taking the threat seriously. He says the security services are implementing anti-terror measures on what he calls a constant basis.

"They are saying simply, reassuring the population, saying simply that they are on alert," he added. "And they are even prepared for some kind of bomb threats in Poland. But they say, rather that they think there might be some attacks against the Polish forces in Iraq, which is much more probable."

A day before the threat to Poland and Bulgaria, another group warned Japanese troops in Iraq that they would be blown up by car bombs, if they did not withdraw from the country. Japan answered that it would press ahead with its humanitarian mission and urged other countries to support Iraqi reconstruction.

A top security official at NATO says the decision by the Philippines to withdraw its small contingent to gain the release last week of a Philippine truck driver kidnapped by militants probably sparked the most recent threats.

Security expert John Tesh says the flurry of warnings is aimed at breaking up the U.S.-led coalition in Iraq.

"I think it's part of a general policy of attempting to split off other members of the coalition, specifically from the United States," added Mr. Tesh. "I think, if you look at some of the kidnappings, for example, that have taken place and the demands that have been made, the demands are being made on the Americans, basically to do things like release prisoners and so forth, and what they're saying is that, unless the United States does something, then they will execute people they have kidnapped from other states that are involved in Iraq."

A Bulgarian truck driver was beheaded this month by extremists in Iraq who demanded that the United States release all Iraqi prisoners. Hopes that a second Bulgarian hostage is still alive are fading.

The United States insists the coalition remains strong, even though Spain pulled its troops out of Iraq shortly after the Madrid bombings in March, and an election there that brought the opposition Socialists to power. But the new Spanish prime minister, Jose Luis Rodriguez Zapatero, an opponent of the war in Iraq, says he made the move to honor a campaign pledge, and insists he is determined to fight terrorism. Three Latin American countries whose small contingents were serving under Spanish command also withdrew their forces.

Italy received a warning a week ago from an al-Qaida-linked group that it would face attacks on its home soil, if Prime Minister Silvio Berlusconi, a strong ally of President Bush, remains in power, but it has shrugged off the threat. Mr. Berlusconi says Italian troops will stay in Iraq, until democracy takes hold.

The Polish and Bulgarian governments have reacted in a similar fashion, despite widespread opposition at home to the presence of their forces in Iraq. Pledging not to bow to the demands of terrorists, they insist that the 2500 Polish and 480 Bulgarian troops will remain there.

Warsaw editor Piotr Stasinski says that, in Poland's case, a terror threat would be counterproductive.

"If there is a threat, some people will say 'no, we wouldn't give in to pressure from terrorists, even if we don't approve of the intervention and the Polish presence there,'" he said.

Security analyst John Tesh agrees that the only possible response to threats is not to give in to them.

"Whatever people in Poland and Bulgaria and Italy think about Iraq and the presence of troops from their country in Iraq, they do not like terrorism," he noted. "And they know that you can't give in. And even though the policy may be unpopular in some quarters, the idea of not giving in to terrorist demands will not be that unpopular."