WASHINGTON (Reuters) - In picking his defense secretary on Thursday, President-elect George W. Bush may have nudged Washington toward a collision with Russia, China and U.S. allies over the thorny issue of national missile defense.

Donald Rumsfeld, who first headed the Pentagon 25 years ago, made clear he considered the United States increasingly vulnerable to ballistic missile attack from countries including North Korea, Iran and Iraq.

"There's no question but that the proliferation of weapons of mass destruction and the delivery systems for them is extensive across the world," Rumsfeld, 68, said after his nomination, which was unexpected. "And I consider that, myself, to be a threat."

He said "a number of nations" were exporting the know-how to field missiles that could hit the United States -- a possible reference to China and North Korea -- making deployment of a missile defense system appear likely.

Bush said he wanted Rumsfeld to "make sure that the missile defense receives the priority we think it must receive in future Pentagon budgets."

Bush, who will be sworn in on Jan. 20, campaigned on a promise to shield the United States and allies from missile attack. He promised to do so even if it meant scrapping the 1972 Anti-Ballistic Missile Treaty with Russia, a keystone of arms control for 30 years.

RUSSIAN OPPOSITION

To date, Russia has opposed all changes to the ABM Treaty. If Washington pulls out of the pact, Moscow has said other deals might unravel, including the two nuclear arms reduction treaties signed by Bush's father, President George Bush, at the end of the Cold War.

"As to missile defense, there's a selling job to do there," the president-elect told reporters, referring to opposition among Democrats in the divided Congress being sworn in next week. A good place to start, he said, was with what some critics deem the alarmist findings of a bipartisan commission that Rumsfeld headed in 1998.

The congressionally chartered, nine-member panel unanimously concluded that U.S. intelligence had underestimated the threat of missiles that could be tipped with nuclear, chemical or biological warheads.

Rumsfeld's panel asserted that "rogue states" could acquire ballistic missiles within "five years of a decision to do so," not the 15 years or more suggested by earlier U.S. intelligence analyzes.

His findings led President Clinton, in his final two years in office, to treat more seriously the plan for a system of radar, rockets, interceptors and battle management computers -- a land-based version of the space-based "Star Wars" antimissile shield pushed by former President Ronald Reagan.

CLINTON DEFERRED DECISION

However, after two of the first three intercept tests failed, Clinton deferred to his successor a decision on whether to start building a radar post in Alaska next spring to deploy a limited defense by 2005 or 2006.

Russia, China and NATO allies have all warned against a unilateral U.S. move to erect a shield and tilt the post-World War Two strategic nuclear balance.

Bush's secretary of state-designate, Colin Powell, disappointed some hawks two weeks ago by promising a "full assessment" of the technology by the new defense secretary.

"Especially worrisome to supporters of missile defense is the prospect that such a review and negotiations may be conducted by people who have, heretofore at least, evinced little enthusiasm for actually deploying a U.S. antimissile defense," wrote Frank Gaffney, a leading advocate.

Gaffney, a Pentagon strategist under Reagan from 1983 to 1987, argued that a drawn-out review would give opponents time to organize and inevitably entangle the issue in the 2002 congressional elections.

Clinton's first CIA director, James Woolsey, who served on the Rumsfeld panel after leaving office, said he was confident Rumsfeld would weigh missile defense options "objectively."

But William Hartung, an arms trade expert at the New York-based World Policy Institute, called Rumsfeld's appointment a "great victory for the Star Wars lobby."

Boeing Co. has a three-year, $2.2 billion contract to tie together the system's main components. Subcontractors include Raytheon Co . on radars and the "kill vehicle," TRW Inc. on command and control and Lockheed Martin Corp. on the initial booster.