WASHINGTON - A number of countries are developing ways to knock out US space systems, threatening vital national interests, the State Department's point man on international security said on Wednesday.

Robert Joseph, undersecretary of state for arms control and international security, did not name any such states but served notice Washington was taking steps to head them off.

"We will seek the best capabilities to protect our space assets by active or passive means," he said in elaborating for the first time publicly on a recent Bush administration revision of US space policy, the first in nearly 10 years.

He referred to such possibilities as maneuvering out of harm's way, redundancy, system "hardening," encryption and rapid frequency changes.

In reply to a question, he added that nothing in US policy ruled out basing weapons in space to defend space assets.

At issue is everything from the Defense Department-run Global Positioning System used for precision navigation and timing signals to spycraft, systems that track missiles and commercial satellites vital for communications.

"The United States is more dependent on space than any other nation," Joseph said. As a result, US space infrastructure could be seen as "a highly lucrative target."

The updated US space policy, released two months ago, rejected a push by China, Russia and others for new arms-control pacts to keep space free of offensive weapons. It outlined a stepped-up drive to guard space assets in light of growing US reliance on them amid reported growing threats.

Joseph declined to comment on published reports citing Donald Kerr, director of the US National Reconnaissance Office, as having said in September a US satellite had been illuminated by a laser in China.

"As a matter of policy, we do not talk about specific threats or vulnerabilities," he told a forum organized by the George C. Marshall Institute, a public policy group.

But he said not all countries could be relied on to pursue exclusively peaceful goals in space.

"A number of countries are exploring and acquiring capabilities to counter, attack and defeat US space systems," he said.

"Given the vital importance of our space assets, foreclosing technical options to defend (them) in order to forestall a hypothetical future arms race in space, is not in the national security interest of the United States," Joseph said.

In reply to another question, he appeared to discount international efforts to keep the United States from developing what could become the first known weapons in space designed specifically to apply force.

"What normally one finds when you strip away the veil on the issue of weaponization of space ... is a desire to constrain US options for the development of our missile defense capabilities," he said.

"I find this quite odd because it is those missile defense capabilities that are designed to counter offensive ballistic missiles," Joseph added.