Secret U.S. diplomatic cables published by the WikiLeaks website reveal that most of the 200 U.S. tactical nuclear weapons based in Europe are located in Belgium, the Netherlands, Germany and Turkey.
NATO spokeswoman Oana Lungescu told VOA the leaking of any classified information, diplomatic or military, was "illegal, irresponsible and dangerous".
She declined to say anything further, saying that as a matter of policy NATO did not comment on classified information.
The countries have long been suspected of housing the warheads, but the leaked cables appear to be confirmation of that information.
Other U.S. diplomatic cables leaked by WikiLeaks say Afghan President Hamid Karzai freed detainees without trial in Afghanistan because they had powerful connections.
Documents released Tuesday by the website included a diplomatic cable from the U.S. embassy in Afghanistan, saying Karzai and his attorney general released dangerous detainees and suspected drug traffickers without them ever facing an Afghan court.
The cable said despite repeated rebukes from U.S. officials in Kabul, the Afghan government allowed the prisoners to go free and some even returned to fighting.
The cable said the Afghans released some 150 prisoners transferred from the custody of the international coalition to Afghanistan. Several of those were described as having powerful friends or relatives.
Afghanistan has not commented on Tuesday's disclosure. On Monday, a spokesman for President Karzai said there were "no surprises" in cables that describe the leader as "extremely weak" and his half brother as a corrupt drug trafficker. Afghanistan says it does not expect those leaks to strain relations with the U.S.
The United States has condemned the WikiLeaks release of more than a quarter-million sensitive U.S. diplomatic cables.
Secretary of State Hillary Clinton said Monday the disclosure is an attack on American foreign policy interests and the international community. She said the United States is taking action to make sure such a breach does not happen again.
Clinton said she wants to make clear that U.S. foreign policy is not set by cables like those that were released, and that U.S. policy is a matter of public record.
WikiLeaks earlier this year published documents on the Afghan war, as well as 400,000 documents on the Iraq war.
The website has not identified the source of the documents. A U.S. Army intelligence analyst, Bradley Manning, was arrested earlier this year and is in military custody awaiting trial for allegedly leaking a 2007 video of a helicopter strike in Iraq and classified diplomatic cables to WikiLeaks.
WikiLeaks was founded by Julian Assange, an Australian citizen and former computer hacker. The highly secretive Assange has been criticized by the Pentagon for his previous releases of confidential material.