For a U.S. military increasingly dependent on sophisticated satellites for communicating, gathering intelligence and guiding missiles, the possibility that those space-based systems could come under attack has become a growing worry -- and the perceived need to defend them ever more urgent. And that, in turn, is reviving fears in some quarters that humanity's conflicts could soon spread beyond Earth's boundaries.

In a speech last week, a senior Bush administration official warned that other nations, and possibly terrorist groups, are "acquiring capabilities to counter, attack and defeat U.S. space systems." As a result, he said, the United States must increase its ability to protect vital space equipment with new technologies and policies.

Elaborating publicly for the first time since the October release of a new national space policy, Undersecretary of State Robert G. Joseph made clear that the administration would react forcefully to any attempt to interfere with U.S. space technology -- whether used by the military or by businesses ranging from paging services and automated teller machines to radio and television providers.

"No nation, no non-state actor, should be under the illusion that the United States will tolerate a denial of our right to the use of space for peaceful purposes," said Joseph, undersecretary for arms control and international security.

"We reserve the right to defend ourselves against hostile attacks and interference with our space assets. We will, therefore, oppose others who wish to use their military capabilities to impede or deny our access to and use of space. We will seek the best capabilities to protect our space assets by active or passive means."

The administration insists that there is no arms race in space, although the United States is the only nation that opposed a recent United Nations call for talks on keeping weapons out of space.

The statement of American resolve in space came against the backdrop of an intensifying debate between those who criticize any push to put weapons in space and others who say the nation cannot afford to let potential adversaries get the upper hand.

Some Democrats and representatives of other nations are becoming more vocal in their concern about the administration's rhetoric and possible plans regarding space defense. Although the 1967 U.N. Outer Space Treaty, signed by the United States, allows only peaceful uses of space, some believe that the United States is moving toward some level of weaponization, especially related to a missile defense system.

Both the new space policy and Joseph's speech "left a lot of room for weaponization of space, which is something that our members have been very concerned about for a while," said Loren Dealy, spokeswoman for the Democratic majority on the House Armed Services Committee. "It also took a very unilateral approach and did not address the issue of multinational agreements to protect satellites that are there."

Sen. Christopher J. Dodd (D-Conn.) earlier criticized the president's new national space policy, saying, "As we deal with the threats to peace and security from the proliferation of land-based weapons, surely we need to think long and hard before creating potential space-based proliferation threats."

Theresa Hitchens, director of the nonpartisan Center for Defense Information, said she found the tone and substance of Joseph's comments last week puzzling.

"It is somewhat ironic that while he kept saying 'There is no arms race in space' -- which says to me no real threat in space -- his whole pitch was how we have to protect our satellites, including using weapons," she said, citing Joseph's mention of "active means" of defending assets. "The truth of the matter is that the most likely threats are from the ground -- jamming, hacking, blowing up a tracking station -- and anti-satellite weapons and/or space-based weapons do nothing to resolve those threats."