ASHINGTON, Feb. 8 - The United States still keeps as many as 480 nuclear weapons at air bases across Europe, more than twice what independent military analysts previously estimated, according to a new study that says the weapons' presence is hurting efforts to curb nuclear proliferation worldwide.

Military officials insisted that the size of the nuclear stockpile in Europe, while classified, was smaller than that. But they acknowledged that it still existed to deter terrorists or nations that could threaten America or its allies with unconventional weapons. The officials also say the stockpile's presence and its long-term fate have caused simmering tensions among senior NATO political and military officials.

The report by the Natural Resources Defense Council, a private group here that advocates arms control and monitors nuclear trends, says short-range nuclear weapons are stored under American control and regulated by secret military agreements at eight bases in Germany, Britain, Italy, Belgium, Turkey and the Netherlands. The bombs are kept under tight security at sites reinforced against attack.

American and allied air forces regularly rehearse nuclear bombing missions at training ranges in Europe in the case a war calls for striking nuclear, chemical or biological weapons sites or command posts in countries that threaten to use unconventional arms, the report states. Military officials confirmed that the training continued as part of prudent military contingency planning.

The findings in the 102-page report, "U.S. Nuclear Weapons in Europe," come as NATO defense ministers, including Defense Secretary Donald H. Rumsfeld, prepare to meet Wednesday and Thursday in Nice, France. An advance copy of the report was provided to The New York Times by the research council.

One topic of discussion is likely to be nuclear proliferation, including Iran's nuclear program, Pentagon and NATO officials said.

Capt. Curry W. Graham, a spokesman for the military's European Command, said the United States still maintained a sizable nuclear arsenal in Europe to support NATO's strategic deterrence mission to "maintain peace and stability in the region." Pentagon policy prohibits the disclosure of the amount or location of American nuclear weapons.

But a senior military official in Europe said in response to the report's findings that the number of American nuclear weapons there was now "around 200," and had been "significantly reduced" in recent years.

The author of the research council report, Hans M. Kristensen, a nuclear arms specialist and consultant for the organization, acknowledged that he did not have the most recent data but said his conclusions were based on recently declassified documents, commercial satellite imagery and other documents. He added that classified documents he obtained as recently as last year showed the nuclear stockpile to be roughly what his new study estimates.

A former senior American officer in Europe said the report's accounting of weapons was "in the ballpark." And a NATO briefing in June 2004 showed the nuclear stockpile in Europe had not changed in more than a decade, suggesting any reductions had taken place quite recently.

A study the council did in 1998 estimated the number of nuclear weapons in Europe at about 150.

The senior military official in Europe would not discuss which countries or targets the weapons could be used against, but military officials in the past have left open the possibility, however remote, of using nuclear arms against targets in so-called rogue nations, including Iran and Syria, if they threatened to use unconventional weapons.

"Militarily, you can't rule out something like a biological threat, so this capability has not been taken off the table," the retired senior American officer said.

There is no proposal to reduce the American nuclear arsenal in Europe, officials said, but the issue has caused strain among the alliance's political and military leaders. "Some allies and U.S. military see a lot of value in going to zero," the senior military official in Europe said. "That said, some allies and U.S. military see value in at least keeping some capability."

Gen. James L. Jones, the head of the European Command and the top NATO commander, has privately told associates that he favors eliminating the American nuclear stockpile in Europe, but has met resistance from some NATO political leaders. The alliance's Nuclear Planning Group is to meet Feb. 17, but it is unclear if the issue will come up then.

Spokesmen at the embassies of several European nations here declined to comment, citing their policy of not discussing American nuclear weapons on their soil.

At the height of the cold war in the early 1970's, the United States had about 7,300 short-range nuclear weapons in Europe to be used as a last resort against a huge ground attack by the numerically superior Soviet military, the report said.

Arms control agreements in the 1980's began to reduce that number, and after the collapse of the Soviet Union, President George H. W. Bush announced in September 1991 that the United States would withdraw all tactical ground-launched and naval nuclear weapons worldwide.

About 1,400 air-delivered nuclear bombs were still left behind, the report says, but that number continued to dwindle over the next decade. The remaining weapons in Europe are B61 bombs, which can be dropped from fighter planes and are typically less powerful than long-range nuclear weapons fired from silos or submarines, the report said.

The research council's report challenges the rationale for keeping short-range nuclear weapons in Europe when the United States has thousands of long-range missiles that could hit any target in a matter of minutes.

Unlike the situation during the cold war, American aircraft are not kept on alert to deploy at a moment's notice. Still, with the United States straining to meet many of its conventional missions in Iraq and Afghanistan, the report asserts that eliminating weapons to be dropped by Air Force F-15's and F-16's could free up fighter-bombers for those missions.