Just days after the Pentagon's latest failure to shoot down one missile with another, key senators warned that the proposed national missile defense (NMD) system was not yet technologically feasible, and that opposition from Russia and China suggested it could actually harm national security.

"This system is not ready for prime time. No president, this one or the next one, unless things change drastically, should in fact deploy this system," said Senator Joseph Biden, the top Democrat on the Senate Foreign Relations Committee.

"There's no way that we can say that this system will be operationally effective, given the failure of this last test, given the technological complexity of this," said Carl Levin, the top Democrat on the Senate Armed Services Committee.

Senate Democratic Minority Leader Tom Daschle told reporters he would urge Clinton, set to make a decision later this year on the 60-billion-dollar program, to let the next administration decide whether to push forward with NMD.

The comments came after the Senate voted 52-48 to shoot down a proposal that would have toughened testing of the missile shield, pitting it against countermeasures that many scientists say would easily render it impotent, including boosting the number of warheads or launching decoys.

"Now the Senate is on record as saying it doesn't matter whether it works or not, we're going to build it any way," Daschle deplored.

The vote reflected the staunch support for the program, which backers say is critical to thwarting attacks by nations like North Korea, Iran or Iraq. Clinton's decision would start the ball rolling towards having an operational NMD by 2005.

Critics -- pointing to recent testing failures -- say NMD is not currently feasible and could destabilize US ties with Russia and China, which say the plan violates the 1972 Anti-Ballistic Missile (ABM) treaty.

"We don't have sufficient information about its technological feasibility, we really don't know what its impact is going to be on ABM, we really don't know its effect on overall national security," said Daschle.

If Clinton does not give the go ahead this year to start construction on the first piece of the system -- a high frequency radar on Shemya island in Alaska -- the Pentagon will be unable to deploy the system by 2005, Pentagon officials said.

Technological concerns include a group of scientists' assertion that any nation technologically able to launch a missile could defeat the missile shield by multiplying the number of warheads or sending up decoys to overwhelm the system.

Of three attempted interceptions, two have failed, falling short of the Pentagon's own minimum criteria of scoring two intercepts before declaring the system ready for deployment in 2005. That is when US intelligence services estimate that North Korea is likely to have long-range missiles capable of striking the United States.