The most senior US diplomat for Asia is seeking an explanation for comments made by the president of the Philippines, Rodrigo Duterte, about his country separating from the US.

Daniel Russel, the assistant secretary of state for east Asian and Pacific affairs, expressed concern over Duterte’s deadly war on drugs and anti-US rhetoric during a meeting with the Philippine foreign secretary, Perfecto Yasay, on Monday.

“I’ve pointed out to secretary Yasay that the succession of controversial statements, comments and a real climate of uncertainty about the Philippines’ intentions has created consternation in a number of countries, not only in mine,” Russel said. “This is not a positive trend.”

The relationship between the Philippines, a former US colony, and Washington has soured significantly since Duterte took office in June, reaching a low when he suggested Barack Obama was a “son of a whore”.

Last week, on the eve of a visit to China, Duterte said: “I announce my separation from the United States … both in military and economics also.”

In a press briefing, the spokesman for the US Department of State, John Kirby, said Washington was “baffled” by the comments and sought “an explanation of exactly what the president meant when he talked about separation from the US”.

While he stated that Russel’s trip had been planned months in advance, Kirby said: “It does give us an opportunity in the context of these comments to try to get a better explanation of what was meant by ‘separation’ and where that’s going.

“I think [Russel’s] conversations will hopefully suffice for the kind of explanation and more detail that we’re seeking.”

Shortly after he arrived in the Philippine capital, Manila, Russel’s team tweeted a photo of Philippine newspaper headlines referring to the separation comment.

Arrived in Manila. Trying to make sense of what we're hearing. Will ask our friends. pic.twitter.com/SBExcGmXPD — EAP Bureau (@USAsiaPacific) October 22, 2016

Duterte visited Beijing last week and has increasingly spoken about friendly ties with China after years of antagonism over a territorial dispute in the South China Sea. Observers say Duterte is looking to China to take over from the US as a major ally for the Philippines.

But Russel said it was a mistake to think improved relations between Manila and Beijing “somehow come at the expense of the United States … This should be addition and not subtraction”.