The U.S. Pentagon and State Department have devised plans to supply Ukraine with antitank missiles and other weaponry and are seeking White House approval, U.S. officials said, as Kiev battles Russia-backed separatists and ties between Moscow and Washington fray.

American military officials and diplomats say the arms, which they characterized as defensive, are meant to deter aggressive actions by Moscow, which the U.S. and others say has provided tanks and other sophisticated armaments as well as military advisers to rebels...