WASHINGTON (AP) _ The Clinton administration today sought to reassure Russia, China and other critics of missile defense that an anti-missile system to protect the United States and its allies would not undermine global security.

Walter Slocombe, the undersecretary of defense, said President Clinton would make a decision ``next summer at the earliest'' whether to order the deployment of a limited national missile defense. By then the administration hopes to win Russia's agreement to modify the 1972 Anti-Ballistic Missile treaty, which prohibits the kind of defensive system the Pentagon is proposing at a cost of about $11 billion.

Russia has steadfastly refused to negotiate changes to the ABM treaty, arguing that amendments that would accommodate the proposed U.S. national missile defense system would ruin the treaty and undermine global security. China, which has a smaller nuclear force than Russia, also opposes the U.S. plan.

``If they persist absolutely in that position, then the United States ... will have to face a very difficult question, which is whether to withdraw from the treaty,'' Slocombe said in remarks at the Center for Strategic and International Studies. He said the Clinton administration would prefer to preserve the ABM pact but will not let Russian objections stand in the way of U.S. national security needs.

He also suggested that Russia would be undermining its own security by refusing to amend the ABM treaty.

``It is not at all clear to me ... that Russia would gain anything from the destruction of the arms control framework,'' Slocombe said. At another point he said, ``The consequences would be difficult for Russia.''

Slocombe's audience included numerous foreign diplomats, including representatives from Russia, China, India, Pakistan, Japan, Korea and several European countries.

The Clinton administration, with the support of Congress, argues that a defensive system is needed to protect all 50 states against a limited attack with ballistic missiles from a ``rogue state'' such as North Korea or Iran, which do not have the full-scale nuclear arsenals of Russia or China.