Amid persistent reports that Philippine President Ferdinand Marcos is ill, the presidential palace said tonight he is well and performing his usual duties.

The statement issued by Assistant Information Secretary Amante Bigornia said Marcos, 67, was in touch last night by telephone with his Defense Minister Juan Ponce Enrile, Information Minister Gregorio Cendana and Acting Armed Forces Chief of Staff Lt. Gen. Fidel Ramos.

But for the sixth straight day, Marcos failed to appear on the television news, which is unusual for a leader who tends to dominate news events here. Despite his 18 years in power, he has not named a successor but has laid out an untried succession plan: to elect a president within 60 days should Marcos die.

The rumors on Marcos' health began circulating two weeks ago and gained currency today when his former information minister, Francisco Tatad, wrote in his column in Business Day newspaper that palace sources said Marcos had undergone a successful operation on Nov. 14. He did not specify the nature of the alleged operation.

Marcos is said to be suffering from a kidney ailment. It has been government policy not to announce an illness of the president since his health noticeably declined four years ago. He dropped out of public view after meeting U.S. Sen. Christopher Dodd (D-Conn.) on Nov. 13.

A similar withdrawal from public view occurred in August 1983. Western diplomats here said they have reason to believe that Marcos is ill again. But they discounted a rumor that he had a kidney transplant.

The public is given the impression that Marcos is still running the government. Daily press releases from the palace refer to presidential directives. Local newspapers have been offered photographs, which appear to be recycled.

Tatad said only Marcos' immediate family had access to the president and his elder daughter Imee was managing the press releases to prevent any bungling. Tatad also said Marcos' wife Imelda earlier had inspected the Heart Center in the suburbs, where he said the operation took place.

[State Department spokesman John Hughes, responding to wire service accounts of Tatad's column mentioning possible treatment of Marcos in the United States, said, "He is certainly not in the United States, to our knowledge." Hughes also knocked down a report that President Reagan and Marcos had a telephone conversation about Marcos coming here for treatment. Records have been checked at the White House and there is "nothing to indicate" the conversation ever took place, he said.]