WASHINGTON (AP) _ The Reagan administration, responding to a report that Iceland will join New Zealand in banning U.S. warships equipped with nuclear weapons, reiterated Wednesday its policy of refusing to confirm or deny the existence of such weapons on Navy vessels.

''We can only state that longstanding U.S. policy on nuclear weapons overseas remains fully in accord with existing bilateral agreements in conformity with NATO defense plans and in agreement with the states directly concerned,'' State Department spokesman Bernard Kalb said.

''There has been no change in U.S. policy to neither confirm nor deny the presence of nuclear weapons aboard U.S. naval vessels,'' he said.

Icelandic Foreign Minister Geir Hallgrimsson was quoted as saying: ''As Iceland excludes nuclear arms generally from its sovereign area, it is a natural conclusion that nuclear-armed ships are barred from entering ports or even sailing in the country's territorial waters.''

U.S. ships, and those of other nations in the North Atlantic Treaty Organization, regularly visit ports in Iceland, which is a NATO member.

New Zealand banned ships equipped with nuclear weapons earlier this year, causing a rupture in the ANZUS alliance.

When Kalb was asked whether nations that ban such visits could still be called U.S. allies, he replied, ''I'm not going to take a jump on that.''

The spokesman also repeated that, despite assertions by the Chinese to the contrary, the United States has given no assurance that U.S. ships scheduled to visit Shanghai will not be carrying nuclear weapons.

No specific date has been set for the visit.