Citing a new strategic relationship with Russia and the need for defenses from terrorist threats, President Bush withdrew from the 1972 Anti-Ballistic Missile Treaty Thursday.

"Today I have given formal notice to Russia, in accordance with the treaty, that the United States of America is withdrawing from this almost 30-year-old treaty," Bush, flanked by Secretary of State Colin Powell, Defense Secretary Donald Rumsfeld, Joint Chiefs Chairman Gen. Richard Myers and national security adviser Condoleezza Rice, told reporters in the Rose Garden.

The U.S. ambassador to Moscow delivered notice of Bush's decision to Russian officials at 4:30 a.m. EST, according to a senior administration official.

Russian President Vladimir Putin responded in a nationally televised address.

"This step was not a surprise for us. However, we consider it a mistake," Putin said, repeating Russia's oft-stated position that the treaty is a cornerstone of world security.

The brief legal document invokes Article 15 of the 29-year-old treaty to give Russia six months' notice of Bush's intentions. The official said Bush has, in effect, pulled out of the treaty with the notification, though the United States cannot conduct missile tests barred by the treaty for six months.

At 9 a.m. EST, formal notice was also given to Ukraine, Kazakstan and Belarus, former Soviet states who signed memoranda of understanding tying them to the pact under the Clinton administration. That action underscored Bush's position that the ABM is a Cold War relic, officials said.

Earlier this week, Bush made his decision to withdraw from the ABM treaty, which was predicated on the "grim theory" of mutually assured destruction, after Russia refused to go along with changes to the treaty that would allow the Pentagon to conduct tests on a national missile defense system.

The president said the reach of global terrorism displayed in the Sept. 11 attacks and the efforts of terrorists to obtain weapons of mass destruction show it's more important than ever to build a missile defense.

"We know that the terrorists and some of those who support them seek the ability to deliver death and destruction to our doorstep via missile, and we must have the freedom and the flexibility to develop effective defenses against those attacks," Bush said.

Bush said that Putin understands that the U.S. withdrawal is not a reaction to U.S.-Russian relations, which are more "hopeful and constructive" than they have ever been.

"One of the signatories — the Soviet Union no longer exists and neither does the hostility that once led both our countries to keep thousands of nuclear weapons on hair-trigger alert, pointed at each other," he said, adding that Russia is in the midst of a transition to a free markets and democracy and that the United States is committed to helping forge strong economic ties and new bonds with NATO countries.

Bush tried to strike a deal with Putin that would allow the United States to move to a new phase of testing in the U.S. missile defense program. Putin had sought authority to sign off on U.S. missile tests, but the request was rejected, administration officials said.

The next scheduled step is the beginning of construction next spring of silos and a testing command center near Fairbanks, Alaska.

Bush softened the blow by agreeing last month to a reduction in nuclear stockpiles from about 6,000 weapons to 1,700-2,200. The Bush administration also intends to cooperate with Russia at least to the extent of informing Moscow of steps being taken to advance the missile-shield program.

That's not likely to stop Russia from taking retaliatory steps. A senior Russian lawmaker predicted Russia will pull out of the Start I and Start II arms reduction treaties.

"We believe that offensive and defensive tools of nuclear deterrence must be linked," said Dmitry Rogozin, chairman of the Duma's international affairs committee, according to Interfax news agency.

But the current war on terrorism may limit the fallout from Russia, China and European allies.  

China's small nuclear arsenal would be neutralized by a U.S. missile defense system, and Europe is worried about the new arms race that could result, as China, Russia and others seek to make themselves less vulnerable.

Russian Prime Minister Mikhail Kasyanov said the decision was regrettable because it undermined global strategic balances — but he was not concerned about Russia's security, though Russia cannot afford to build its own defense system.

"Russia can be unconcerned with its defense systems," said Kasyanov, who was in Brazil for a two-day visit. "Maybe other nations should be concerned if the United States chooses to abandon the Anti-Ballistic Missile Treaty."

The timing of the announcement works in the president's favor domestically. Congress cannot block Bush's decision, and the president's strong public approval rating also means critics aren't likely to be as vocal as they might have been.

That didn't stop Senate Democrats from complaining that a unilateral withdrawal will lead to a new arms race.

"About eight months ago they were taking about weaponizing space," Biden said Wednesday. "God help us when that moment comes," said Sen. Joe Biden, D-Del., chairman of the Senate Foreign Relations Committee. 

Fox News' Wendell Goler and the Associated Press contributed to this report.