Washington has been boosting its backing for Philippine counter-terror efforts since supporters of the Islamic State group seized parts of the southern city of Marawi last year, sparking a deadly five-month battle.

The unmanned aerial vehicle system, which includes six drones and is worth $13.2 million (10.7 million euros), is the latest US military assistance to Philippine troops.

"Assets like the ScanEagle will significantly improve the (Philippine military's) ability to detect terrorist activities, piracy activities, territory encroachment," US Ambassador to Manila Sung Kim told reporters, referring to the drones.

Philippine Defence Secretary Delfin Lorenzana said the system, equipped with cameras and aircraft that can fly for 24 hours, would support operations against militants in the south.

The southern region of Mindanao is home to several pro-IS groups, including those who attacked Marawi in May last year in fighting that claimed more than 1,100 lives and reduced large parts of the city to rubble.

These new drones are to be used for reconnaissance missions in defence, humanitarian assistance and disaster response, Lorenzana said.

"With a number of security issues confronting our country today, there is a need to upgrade our nation's armed forces," he added.

President Rodrigo Duterte had sought to loosen the Philippines' 70-year alliance with the United States in favour of closer ties with China and Russia.

His anger was partly driven by American criticism of his drug war, which has seen police kill thousands of people and prompted an International Criminal Court preliminary examination.

Although relations have improved under US President Donald Trump, who has praised Duterte for his drug war, the Philippine leader has increasingly turned to Beijing and Moscow to boost one of Asia's weakest armed forces.

Duterte last month cancelled a deal to buy helicopters from Canada after Ottawa ordered a review over human rights concerns.

"Do not buy anymore from Canada and the US because there is always a condition attached," Duterte said at the time.

A US embassy spokeswoman said Washington "is committed to the rule of law and respect for human rights, and we urge other governments to do the same".

The surveillance system is aimed at bolstering counter-terrorism operations and disaster relief, she added.