MANILA -- Another week, another rant from Philippine President Rodrigo Duterte, who continued his anti-U.S. tirade while in Japan last week. The remarks are unlikely to affect the tremendous popularity Duterte enjoys at home, but will add to growing international concerns.

The U.S. withdrew its military installations from the country by 1992, due in part to local resentment. With the U.S. presence in Asia changing, China has stepped up maritime expansion in the region.

In 2014, the U.S. and the Philippines concluded an agreement to bolster the two countries' military alliance, effectively paving the way for the return of U.S. troops to the Philippines.

People close to Duterte say his stance can be traced back to his time as mayor of Davao on the southern island of Mindanao.

Back in 2002, an explosion occurred in a hotel room in the city, injuring it's American occupant, Michael Meiring. Local police concluded that the blast was caused by explosives in Meiring's possession.

A few days later, men carrying FBI badges removed Meiring from the hospital where he was being treated and allowed him to leave the country without permission from local authorities, according to Duterte. The U.S. Embassy is said to have arranged the flight carrying Meiring, an alleged FBI agent himself.

The American ambassador to the Philippines at that time promised a thorough investigation and that a report would be submitted to Duterte, the Philippine leader told reporters on Oct. 21 before his trip to Japan. The report never materialized, which constituted an insult to the Philippines, he added.

Some analysts trace Duterte's anti-American attitude even further. As a student in the late 1960s and early 1970s, he is said to have belonged to a leftist group, when anti-Vietnam War movements across the world were in full-swing.

Jose Maria Sison, the founder of the Communist Party of the Philippines, was one of his teachers at college. The Duterte cabinet also includes certain members recommended by the communist party.