A senior State Department official leaves today on an unannounced trip to the Philippines amid continuing U.S. concern about the physical and political health of President Ferdinand E. Marcos.

The trip of Assistant Secretary of State Paul D. Wolfowitz is officially described as a routine three-day stopover before an international conference in Bangkok.

State Department sources said, however, that the Philippines stopover is the most urgent reason for the trip. They said Wolfowitz will assess the condition of this troubled allied country and prepare to make recommendations regarding future U.S. aid levels and other policies.

Concern about Marcos' physical health was intensified when he dropped out of sight for three weeks in November. Reports from U.S. diplomats in the area indicate that they do not believe that Marcos' presidency can last for more than two years, at most, due to his declining health, according to official sources.

The political health of the Marcos regime also continues to be fragile, according to reports reaching Washington. One indication of this was the signing of a unity pact Dec. 26 by representatives of a diverse group of Philippine opposition leaders.

A leading opposition figure, former senator Jovito Salonga, held an unpublicized meeting yesterday with Wolfowitz and Undersecretary of State Michael H. Armacost, the department's third-ranking official and a former U.S. ambassador to the Philippines. Salonga, president of the Liberal Party, recently announced his intention to return home after a stay of nearly four years in this country.

The former senator, who faces possible arrest on subversion charges when he returns, said Armacost and Wolfowitz made no attempt to dissuade him from his planned return to Manila Jan. 21.

Asked what U.S. policy should be under the circumstances, he said that "the least the United States should do is not carry out increased military and economic aid. That would signal that there is a change in the U.S. position."

Salonga was described as the latest in a parade of Philippine opposition and government figures who have come to Washington for discussions in the last several months. Wolfowitz is expected to see a broad spectrum of Filipinos, including Marcos and Cardinal Jaime Sin, the Roman Catholic leader who has been at odds with the government.

State Department sources said that, after interagency consideration that began last fall, the administration has decided to consider only "modest" new aid to the Philippines in the next budget year.

The idea of a large-scale new program, dubbed by some as a "Marshall Plan for the Philippines," has been put aside due to the situation in Manila and U.S. budgetary stringency, according to the sources.

Wolfowitz is expected to underscore U.S. support for vigorous implementation of economic policies accepted by the Marcos government in recent negotiations with the International Monetary Fund. One official said Washington fears that a major new economic aid program could dissipate pressures on Manila to follow through on painful economic measures worked out by the IMF.

Another subject of Wolfowitz's inquiry will be change within the Philippine military following the "temporary" relief from duty in late October of Gen. Fabian Ver, armed forces chief of staff, who is alleged to have been involved in the murder of opposition leader Benigno Aquino.

Assistant Secretary of Defense Richard L. Armitage, on a separate survey mission to the Philippines, met with Marcos yesterday, according to reports from Manila. Armitage is said by officials to be urging extensive reforms in the Philippine military.