President Reagan said yesterday the United States is ready to resume high-level talks with Nicaragua's Marxist Sandinista government in the context of the Central American peace process as soon as the Sandinistas begin indirect negotiations with the U.S.-supported contras.

Secretary of State George P. Shultz, elaborating on Reagan's announcement to foreign ministers of the 31-nation Organization of American States, went further. He hinted to reporters that if the Sandinistas take immediate, substantive steps toward democratization, talks with the United States could begin later this week after Nicaraguan President Daniel Ortega arrives Wednesday to address the annual OAS Assembly.

The administration's new flexibility about dealing directly with the Sandinistas on regional security issues capped a process that began last week when the five-nation Central American peace agreement, signed in Guatemala last Aug. 7, was scheduled to go into effect. Ortega, bowing to pressure from the other four countries in the agreement, said he is willing to negotiate indirectly with the contras on a cease-fire through Nicaragua's Roman Catholic primate, Cardinal Miguel Obando y Bravo.

Over the weekend, senior U.S. officials signaled that if such negotiations begin and show signs of seriousness, the United States would resume direct talks with Nicaragua that were broken off in 1984. Yesterday, Reagan used the opening of the OAS Assembly to make the offer official. He appeared at a luncheon for the ministers hosted by Shultz at the State Department and said:

"When serious negotiations between the Sandinistas and the freedom fighters, under the mediation of Cardinal Obando, are under way, Secretary Shultz will be ready to meet jointly with the foreign ministers of all five Central American nations, including the Sandinistas' representatives."

Earlier, following the assembly's opening ceremonies at the Pan American Union, Shultz fielded questions about whether he is willing to meet Ortega by saying the United States would be "prepared to respond in a regional setting" if Nicaragua shows "motion" on requirements of the Guatemala accords such as negotiating a cease-fire in its civil war, releasing thousands of political prisoners and restoring freedom of the press.

When a reporter remarked that such actions could not be taken in the 48 hours before Ortega's scheduled arrival here Wednesday, Shultz replied:

"I think you're categorically wrong. It doesn't take long to let people out of jail. All you do is unlock the doors. It doesn't take long to approve applications for publication. All you have to do is check the 'approve' box. It doesn't take long for Radio Catolica {barred by the Sandinistas from broadcasting news and commentary} to be able to make comments on the issues of the day. All you have to do is say 'okay.'

"So it can happen instantaneously," Shultz concluded.

It was not immediately clear whether the administration's offer actually will lead to U.S.-Nicaraguan talks. However, it did seem to mark a new turn in the administration's zigzagging attitude toward the Guatemala agreement.

Initially, Reagan joined with House Speaker Jim Wright (D-Tex.) last August to sponsor a joint Central America peace initiative. However, their plan was superseded a few days later when the presidents of Nicaragua, Costa Rica, El Salvador, Guatemala and Honduras met in Guatemala to sign a different accord worked out primarily by Costa Rican President Oscar Arias.

Wright immediately became an enthusiastic supporter of the Arias plan. But the administration was unhappy with the plan's failure to impose tough conditions on Nicaragua for cutting military ties to Cuba and the Soviet Union.

The administration later insisted it would ask Congress for $270 million in new military aid for the contras. But opposition from Wright and other members of Congress has forced the administration to defer its aid request at least until January, when the five presidents are to assess whether the agreement has been implemented successfully.

More recently, the administration changed tactics. It prodded the other four presidents to pressure Ortega to deal with the contras; and Reagan's offer to match movement by the Sandinistas with renewed U.S.-Nicaraguan talks appears to reflect a carrot-and-stick approach toward Managua.

However, in his speech yesterday, Reagan carefully recited his familiar charges that the Sandinistas are practicing repression within Nicaragua and spreading communist subversion in neighboring states. He also insisted that his offer of a Shultz meeting with the Sandinistas does not imply any lessening of support for the contras.

"Before such a meeting and throughout this period, we will consult closely with the freedom fighters, for the key to democracy and peace in the region is freedom and national reconciliation in Nicaragua," Reagan said. "Regional negotiations including the United States can be a helpful adjunct to negotiations among the Central American nations and between the Sandinistas and the freedom fighters. They cannot be a substitute."

Wright said yesterday that he had declined for a second time an invitation from Nicaraguan government representatives including Foreign Minister Miguel d'Escoto to serve as intermediary in talks with the contras. Instead, Wright said, he told the Nicaraguans that he believes Obando is "amply able" to play that role.

Applause for Reagan's offer came last night from Sen. Christopher J. Dodd (D-Conn.), chairman of the Senate watchdog committee on the peace plan and a frequent critic of administration Central America policy. Returning from a three-day trip to the region, Dodd said, "I think it's very positive and very productive. The president has correctly framed the U.S. role in the context of the Central American peace accords."