Secretary of State George P. Shultz met with Central American ambassadors yesterday to discuss a Costa Rican plan for regional peace. But, despite assurances that the United States "welcomes" the initiative, it was clear that the Reagan administration still objects to the plan's principal features.

Costa Rican President Oscar Arias' plan for Nicaragua, El Salvador, Guatemala, Honduras and Costa Rica, calls for a cutoff of outside military aid, a cease-fire in regional guerrilla wars, immediate negotiations in Nicaragua between the Sandinista government and its opposition to open up the political process, and a cutoff of U.S. aid to the contra rebels fighting the Sandinistas.

The administration, strongly distrustful of the Sandinistas, is unwilling to disarm the contras unless some form of pluralistic democracy is in place in Nicaragua. In the U.S. view, continuing contra pressure is necessary to force the Sandinistas into accepting that condition.

The U.S. opposition is believed a major factor in the abrupt postponement from June to Aug. 6 of a meeting on the plan by Central American presidents. Last month, Arias was summoned from Indianapolis where he was on a private visit, to the White House, where President Reagan detailed U.S. objections to his plan in a meeting that Costa Rican officials said was "sharp, tense and blunt."

This rekindled charges the administration is interested only in military solutions to Central America's disputes. But Shultz assured the ambassadors of Costa Rica, Guatemala, Honduras and El Salvador at the State Department yesterday that "they have the full support of the United States" in seeking a negotiated peace.

But Costa Rican Ambassador Guido Fernandez later made clear to reporters his belief that U.S. opposition remains unyielding. While insisting that everyone agrees on the goal, Fernandez conceded, "in the means we have a different approach."

Shultz stressed the importance of the word "simultaneous" in any consideration of the Arias plan -- an indication that the United States still insists that support for the contras be ended only when the Sandinistas permit multiparty democracy.

In a related move, Philip C. Habib, Reagan's special envoy for Central America, denied yesterday that the administration seeks to "torpedo" the Arias plan. Testifying before the House Foreign Affairs subcommittee on hemispheric affairs, Habib dismissed as "nonsense" the idea the administration is hostile to Arias and his proposal. But he also said the plan contains "certain deficiencies and omissions that should be corrected."