The International Court of Justice decided unanimously today to accept Nicaragua's suit demanding a halt in U.S. support for "military and paramilitary activities" against the Sandinista government.

The 16-judge panel, also known as the World Court, handed down the interim decision after rejecting by a 15-to-1 vote the U.S. argument that the court did not hold proper jurisdiction to try the case. An American, Stephen Schwebel, was the lone dissenter.

The judgment, which opens the way for further hearings in which Nicaragua is likely to press for compensation, came as another blow to the Reagan administration's policy of supporting opponents of the Sandinista government.

"We are deeply disappointed by the court's decision," said Davis Robinson, the State Department legal adviser handling the case. "We think we presented compelling arguments why this is not the appropriate forum for resolving complex conflicts in Central America."

Carlos Arguello Gomez, the chief counsel for the Nicaraguan government, said he was "happy and hopeful that the court's decision will open a new avenue for Nicaragua's search for peace."

Arguello said that the United States "will become regarded as an international delinquent" if it refuses to abide by the findings of the court.

In Washington, State Department spokesman Alan Romberg, said at the daily State Department news briefing Monday that the United States was "disappointed with the court's decision" and continues to believe "that the court is not the proper forum, either as a matter of law or for helping to achieve a peaceful resolution of the conflict in the region."

[Later, a State Department official said the United States would not decide whether to accept the court's ruling and participate in further sessions until U.S. officials have had a chance to read the opinion, which has not been received in Washington yet.]

In May, the World Court passed another interim ruling calling upon the United States to "cease and desist" in all military activity against Nicaragua after that country sought condemnation of the U.S. role in mining its ports and harbors.

Nicaragua filed its suit against the United States with the World Court on April 9, to seek an end to all U.S. involvement in military actions aimed at toppling its government, along with fair compensation for the damage wrought by anti-Sandinista guerrillas.

Three days earlier, Secretary of State George P. Shultz had sent a letter to the U.N. secretary general declaring that the United States would not accept compulsory jurisdiction of the court in any dispute involving Central American states for the next two years.

U.S. lawyers argued that this suspension nullified, for the purposes of Nicaragua's suit, any American acceptance of the court's jurisdiction. This authority is recognized by only 45 of the more than 160 states that have signed the United Nations Charter, which established the judicial body after World War II.

But U.S. sources admitted later that the Shultz letter was probably a tactical blunder because it angered several judges and deepened antipathy toward the U.S. legal position in the dispute with Nicaragua.

The United States signed a declaration accepting the court's compulsory jurisdiction in 1946. Since the termination clause of that pact requires six months' notice, the court found that the U.S. renunciation sent only three days prior to the filing of Nicaragua's suit could not override the earlier obligation accepting jurisdiction.

The U.S. legal strategy focused on trying to prove that Nicaragua never deposited instruments of ratification with the court accepting its jurisdiction. The United States argued that Nicaragua therefore failed to justify its right to bring the suit.

In seeking dismissal of the suit, the United States also insisted that the court was not the appropriate international forum to consider a political case that should more properly be within the domain of the U.N. Security Council.

The court also accepted, by a 14-to-2 majority, Nicaragua's secondary argument that Washington and Managua are bound by a 1956 "treaty of friendship, commerce and navigation" to take bilateral disputes that cannot be resolved by diplomacy to the World Court.

Schwebel dissented again, saying the treaty could not be invoked because it was purely commercial.

The court defended its right to hear the case by citing previous instances of political conflicts that were handled in parallel with the U.N. Security Council.

The judges noted that the court found in favor of the United States in a suit brought by the Carter administration against Iran in 1980 for seizing the American hostages.