The most senior U.S. diplomat for Asia has arrived in the Philippines and will meet the country's foreign minister on Monday amid confusion over the status of ties between the two long-time allies, said the U.S. embassy in Manila.

The visit by U.S. Assistant Secretary of State for East Asian and Pacific Affair Daniel Russel is part of a three-nation swing through Southeast Asia that also includes Thailand and Cambodia, the U.S. State Department announced on its website.

Russel arrives as confusion lingers over Manila's ties with Washington, with President Rodrigo Duterte saying on Saturday the United States remains the Philippine's "closest friend", after earlier provoking alarm by announcing his country's "separation" from the United States and realignment with China.

The United States is one of the Philippine's largest trading partners and its closest security ally for about 70 years.

Russel was to meet with Philippine Foreign Affairs Secretary Perfecto Yasay, U.S. embassy press attache Molly Koscina told Reuters, without giving details about issues to be discussed.

A Philippine foreign affairs official said Russel and Yasay will discuss the overall bilateral relations of the two allies following Duterte's "separation" statement in China last week.

"The U.S. wants a clarification from the Philippines and what direction the alliance will take," the official told Reuters.

"I think the U.S. is confused. It is getting mixed signals from Manila," the Philippine official said, requesting anonymity because he was not authorized to speak to the media.

Russel is expected to stay in Manila until Tuesday to meet with government officials and have lunch with Young Southeast Asian Leaders Initiative (YSEALI) participants, before leaving for Thailand.

He will be in Thailand until Wednesday to convey his condolences on the passing of His Majesty King Bhumibol Adulyadej and meet with Thai officials to discuss bilateral relations and regional issues.

From Thailand, he will fly to Cambodia to meet with government and opposition party officials, civil society organizations, and address a group of YSEALI members.