HONG KONG — Philippine officials said Monday that President Rodrigo Duterte planned to declare a marine sanctuary and no-fishing zone at a lagoon within Scarborough Shoal, a reef China seized in 2012.

The announcement followed Mr. Duterte’s meeting with President Xi Jinping of China on the sidelines of the Asia-Pacific Economic Cooperation summit meeting in Peru over the weekend. It was unclear whether the plan had Mr. Xi’s backing; the Philippine national security adviser, Hermogenes Esperon Jr., said in a statement on Monday that creating the proposed sanctuary was “a unilateral action.”

The plan comes about four months after the Philippines largely won an international arbitration ruling that had challenged China’s seizure of Scarborough Shoal. It also comes as Mr. Duterte has tried to reset frayed relations with China and has publicly questioned his country’s longstanding ties to the United States.

Mr. Duterte’s communications secretary, Martin Andandar, quoted Mr. Xi as having called for a “favorable environment” at Scarborough Shoal, which both countries claim. The Chinese Ministry of Foreign Affairs did not respond to a faxed request for comment on Monday.

Experts said Monday that it would be difficult to assess the feasibility of Mr. Duterte’s plan without further details, and that a crucial question was whether China would be involved in the implementation or enforcement of the proposed sanctuary.