The Reagan administration sent a high-ranking Pentagon official on a secret mission to Panama last week to press its strongman, Gen. Manuel Antonio Noriega, to step down and allow free elections in the country, State Department and congressional sources said yesterday.

The official, Richard L. Armitage, assistant secretary of defense for international security affairs, held what one U.S. official called "a lengthy session" with Noriega early last week to urge him to withdraw from politics.

Armitage was picked to deliver the administration's strongest direct message to date to Noriega because the Panamanian strongman is a "military man" and Washington wanted "the most effective interlocutor possible," the official said.

Access by U.S. officials to Noriega, who has resisted mounting U.S. pressure to resign, "has not been extensive" recently and the administration wanted to hold a "face-to-face meeting" to make sure he "knows this is our policy," the source said. No details were available yesterday on what Armitage reported Noriega's response to have been.

One source compared the Armitage mission to that of then-Sen. Paul Laxalt (R-Nev.), whom President Reagan sent as his personal envoy to the Philippines in October 1985 to urge President Ferdinand Marcos to hold a free and fair election. The voting, held in February 1986, resulted in Marcos' defeat by Corazon Aquino.

State Department spokesman Charles E. Redman confirmed yesterday that Armitage had visited Panama last week and said the purpose of the mission was "to express U.S. government concern over developments in Panama over the last six months."

One non-U.S. government source said he understands that the Reagan administration favors a reform program that would include Noriega's retirement by April as armed forces chief, as well as the withdrawal of his close military associates, the holding of democratic elections and improvements in the judiciary.

Redman refused to confirm reports that the purpose of Armitage's mission was similar to that of Laxalt's in 1985 or that the Pentagon official had pressed Noriega to resign.

However, Redman said "we've long believed that Panama should join the democratic current in Latin America and move to a situation where the military has a much reduced role in politics.

"We support a democratic system based on credible elections and respect for human and political rights," he added.

The administration, Redman said, "believes, however, that Panama's problems must be resolved by the Panamanians themselves. The solution to their problems will be decided in Panama."

Congressional sources said they understood that administration officials were still assessing Armitage's mission and had not yet been able to determine its effect.

They said the idea for the visit had been brought up in the fall during a meeting between State Department officials and several members of the Senate Foreign Relations subcommittee on Western Hemisphere affairs, including its chairman and ranking minority member, Sens. Christopher J. Dodd (D-Conn.) and Richard G. Lugar (R-Ind.).

At the meeting, the senators suggested that the administration send someone like the U.S. ambassador to the United Nations, Vernon A. Walters, or a member of the Joint Chiefs of Staff to deliver the message that Noriega should step down and allow free elections to be held, the sources said.

It was unclear why the administration chose Armitage instead.

U.S. concern about Noriega increased with reports that he has made overtures to Libya and the Soviet Union and decided to expel employees of the U.S. Agency for International Development.

Noriega last month sent a delegation to ask Libyan leader Moammar Gadhafi for a $200 million loan to help offset the loss of U.S. aid and a drop of $8 billion in Panama's bank reserves.

The Reagan administration froze most of $26 million in U.S. aid to Panama in early July. Last month, Congress voted to cut off all U.S. aid except humanitarian and disaster relief.