Philippines President Rodrigo Duterte said Wednesday he will soon end joint military exercises with the United States, a move that could further dampen relations with Manila's longtime ally after the controversial leader branded Barack Obama a "son of a whore".

Speaking to a raucous crowd at an upscale Hanoi hotel where he kicked off his two-day visit to Vietnam, the foul-mouthed leader said next month's military drills would be the last.

"I will serve notice to you now, that this will be the last military exercise, jointly Philippines-US, the last one," he said in a rambling speech to several hundred Vietnam-based Filipinos.

The Philippine-US Amphibious Landing Exercises (Phiblex) are set to take place from October 4 to 12 in the Philippines, which follow the larger annual Balikatan exercises in April involving more than 8,000 forces from both sides.

The allies have quarrelled in recent weeks, with Duterte vowing to eject US special forces from the country's troubled south, just days after he insulted President Obama at a regional leaders summit in Laos.

The 71-year-old leader has also come under fire for his bloody war on crime that has claimed more than 3,700 lives since he took office in June, including by Obama who has said he must conduct his crime war "the right way" by protecting human rights.

Western governments and rights groups have raised concerns about a breakdown in the rule of law over Duterte's crackdown on crime, a criticism the leader laughed off on Wednesday.

"I am the favourite whipping boy now of the human rights (groups) all over the world," he told the crowd, which gave him a rock star welcome.

Defence ties between the United States and the Philippines date back to 1951, and annual joint military exercises have been a pillar of the alliance.

Duterte will meet with Vietnam's political top brass Thursday to discuss maritime freedom and boosting economic and defence ties. Both Manila and Hanoi are locked in separate disputes with powerhouse Beijing over territory in the South China Sea.