The decades-old tradition appeared headed for the history books last year as a newly-elected Duterte pivoted toward China -- and away from long-time ally the US.

But the number of troops taking part in the drills has increased by a third from last year to 8,000, a return to figures seen in years past when the exercises served as a thinly-veiled deterrent to a rising China.

The reason for Duterte's change of heart on the two-week drills codenamed "Balikatan", or "Shoulder-to-Shoulder", may be down to what experts see as careful efforts by the Filipino military to restrain their unpredictable president.

"The fact it's being done under this administration means they (Duterte's government) now have a better understanding of the security equation," political analyst Victor Andres Manhit told AFP.

Though the bulked-up manoeuvres -- including a live-fire component that was dropped last year -- took place on a naval base just 180 kilometres (110 miles) east of the Filipino-claimed Scarborough Shoal that China has controlled since 2012, the drill's leaders barely mentioned Beijing.

"We are an island nation. That's why we need to improve our capabilities on amphibious operations," Philippine Major-General Emmanuel Salamat told reporters.

"We're not concerned about Scarborough. We're concerned about what we're doing here."

China claims most of the South China Sea, a strategic waterway believed to harbour significant oil and natural gas deposits, but their assertion was ruled illegal in 2016 after Duterte's predecessor Benigno Aquino filed suit before an international maritime tribunal.

Duterte has since reversed course and set the ruling aside, along with long-simmering friction over competing claims to the waters, in order to court Chinese trade and investment.

He has also cut two major annual naval exercises with the US and last year reduced the Balikatan contingent to 5,400 American and Filipino troops.

The decision came at a low point for US-Philippine relations, when Duterte hurled insults at the American ambassador to Manila and served notice that the 2017 edition would "be the last military exercise" with the United States.

Much of Duterte's ire appeared to have been triggered by American criticism of his deadly anti-drug crackdown, which has claimed thousands of lives and been condemned by rights groups as a possible crime against humanity.

The renewed ramping up of the US-Philippine war games coincides with improving ties with Washington under President Donald Trump, who has said drug dealers should face the death penalty.

Analysts said Philippine Defence Secretary Delfin Lorenzana, a retired general and ex-defence attache to Washington, and other advisers had helped moderate Duterte's position.

"He (Lorenzana) could handle, in a very effective way, most of the president's biases without ruffling feathers," said Roilo Golez, a former national security adviser of ex-Philippine leader Gloria Arroyo.

US Marine Lieutenant-Colonel Daniel Gaskell, who took part in the landing exercise Wednesday, sidestepped media questions on Duterte's anti-US claims.

"We are welcomed here, walking in the airport, throughout your country, (but also) by the Filipino armed forces who we've interacted with," he said.

"So we're really happy to be here."