The U.S. government should replace more than 1,000 irradiation machines used in hospitals and research facilities because terrorists could use the radioactive materials inside to make a "dirty" bomb, a government advisory panel has concluded.

"Any one of these 1,000-plus sources could shut down 25 square kilometers, anywhere in the United States, for 40-plus years," according to panel documents obtained by The Associated Press.

The machines are in relatively unprotected locations such as hospitals and research facilities all over the country, and may be a tempting source of radioactive materials for terrorists who want bombs that explode and disperse radioactive debris over a large area, rendering it uninhabitable, the board found.

The irradiators contain Cesium-137, one of the most dangerous and long-lasting radioactive materials. They are used for radiation therapy and to sterilize blood and food.

Swapping the Cesium irradiators for X-ray machines or irradiators that use other materials would cost about $200 million over five years, but it would take the most accessible source of dangerous radioactive material inside the United States "off the table" for terrorists, the panel says.

The recommendation is part of an as-yet-unreleased report that describes how unfriendly nations or terrorist groups could undermine the computers and satellites the U.S. military relies on and attack the United States with radiological or biological weapons or blackmail the U.S. government with a threat of a nuclear detonation, all while manipulating world opinion against the United States in the media and on the Internet.

The report comes from the Defense Science Board, a panel of retired military and CIA officials and defense industry experts who offer the Pentagon possible solutions to actual and potential national security problems. It is expected to be released late this year.

The board wants the Pentagon to create a joint military force able to locate and seize illicit nuclear materials and weapons when they are still in transit, and to safely destroy nuclear weapons captured from terrorists or defeated states.

It says U.S. intelligence has failed to determine what countries or groups are developing or trying to obtain nuclear, radiological and biological weapons and how and when they are likely to use them.

"No adversary can exercise all options; but we don't know which options they can exercise," the documents state.

The report recommends creating "unfettered X-treme intelligence teams" to improve the "poor intelligence community posture." Exactly what the teams would do is classified.

The board advocates diplomacy and trying to influence world opinion so the United States is less likely to be attacked or lured into a foreign war it might not win.

"We are unprepared," state the documents. "At best we will be deterred. Worse, we will enter the fray and then quit when we appreciate the cost of success. Instruments of national power other than the military, such as strategic communication, will assume greater importance."

The U.S government should be promoting universally accepted values like human dignity, economic well-being, health care and education rather than "democracy" and "freedom," the panel states.

"What we say is often not what others may hear — concepts such as 'democracy,' 'rule of law' and 'freedom' have different meanings in different cultures and at different stages of their development," the documents state. "It is about them, not only about us."

It recommends that the State Department spend $250 million over five years to create an independent "Center for Global Engagement" to conduct opinion research and analyses on media and culture that the government can use to design projects and messages that will advance those values.

It also recommends deploying more hospital ships for medical and humanitarian relief; releasing spy imagery to help other countries in crop management, weather forecasting, and environmental studies; and adopting policies that will help create jobs in key strategic nations such as Lebanon, Pakistan and Iraq.