Secretary of State George P. Shultz and Soviet Foreign Minister Eduard A. Shevardnadze opened a new communications center Tuesday to reduce the risk of nuclear war, but they remain at odds over the conflicts in Central America and Afghanistan.

The ribbon-cutting ceremony in a tightly guarded room on the seventh floor of the State Department puts into effect an agreement for superpower cooperation that is in contrast to the U.S. and Soviet regional rivalries.The Reagan administration has supported rebels in Nicaragua and Afghanistan who are trying to topple Soviet-backed leftist governments. Shultz wants Moscow to stop its military aid, while Shevardnadze voiced "some serious concern" with U.S. policies in Central America. But they put aside their differences for Tuesday's ceremony.

From the new center, the United States will have a satellite link to a similar center in Moscow over which full texts and graphics can be transmitted rapidly. The idea is to prevent a nuclear war due to miscalculation or accident. Shultz and Shevardnadze reached agreement last September on the link, which supplements the telecommunications "hotline."

It falls short of the crisis control centers Sens. Sam Nunn, D-Ga., and John Warner, R-Va., the chief congressional sponsors, had hoped to establish in the U.S. and Soviet capitals.

The windowless room will be manned around-the-clock by an American communications specialist and a Russian-speaking officer. Twin steel security doors restrict access.

During his talks here, Shevardnadze was expected to offer May 25-28 as the dates for President Reagan to visit Moscow for his fourth summit meeting with General Secretary Mikhail S. Gorbachev. U.S. acceptance is virtually certain.

The projected centerpiece for the meeting is a treaty to slash U.S. and Soviet arsenals of long-range nuclear arms by 30 percent to 50 percent.

While Shultz said Sunday the agreement was not yet "in the bag," U.S. and Soviet experts will try to square drafts of their treaty proposals during Shevardnadze's three-day visit. Shultz also said the president is prepared to meet with Gorbachev

even if the treaty isn't ready.

But the wars in Nicaragua and Afghanistan cast a cloud over the summit preparations.

Assistant Secretary of State Rozanne L. Ridgway and State Department spokesman Charles E. Redman accused Moscow on Monday of an unceasing supply of weapons to the leftist government in Nicaragua.

Apparently referring also to the reported supply of Soviet rifles and other arms to embattled Panamanian strongman Manuel Antonio Noriega from Cuba, Ridgway said: "What we need now is an end to the continued Soviet provision of these high levels of military equipment."

Redman, meanwhile, accused the Soviets of shipping more than 3,000 tons of equipment to Nicaragua in January and February alone.

He said that for a number of years, the Soviets have supplied Nicaragua with $500 million to $600 million in assistance annually. "I certainly don't see it as an issue heating up," Redman said.