VIENTIANE: Declaring a "moral obligation" to heal the wounds of a secret war, US President Barack Obama on Tuesday pledged help to clear away the 80 million unexploded bombs the US dropped on Laos a generation ago — more than 10 for every one of the country's 7 million people.

Half a century ago, the United States turned Laos into history's most heavily bombed country, raining down some two million tons of ordnance in a covert, nine-year chapter of the Vietnam War. The first US president to set foot in Laos while in office, Obama lamented that many Americans remain unaware of the "painful legacy" left behind by a bombardment that claims lives and limbs to this day.

"The remnants of war continue to shatter lives here in Laos," Obama said before an audience of students, businessmen and orange-robed Buddhist monks who held up cellphones to snap photos of the American president. "Even as we continue to deal with the past, our new partnership is focused on the future," he said.

To that end, Obama announced the US would double its spending on bomb-clearing efforts to $90 million over three years — a relatively small sum for the US but a significant investment for a small country in one of the poorer corners of the world. Obama plans to put a human face on the issue when he meets Wednesday in Vientiane with survivors of bombs that America dropped.

The president did not come to apologise. Instead, he called the conflict a reminder that "whatever the cause, whatever our intentions, war inflicts a terrible toll — especially on innocent men, women and children."

Thanks to global cleanup efforts, casualties from tennis ball-sized "bombies" that still litter the Laotian countryside have plummeted from hundreds to dozens per year. But aid groups say far more help is needed. Of all the provinces in landlocked Laos, only one has a comprehensive system to care for bomb survivors.

"We're incredibly proud of the progress the sector has made over the last five years in terms of the decline in casualties and new victims," said Channapha Khamvongsa of the nonprofit Legacies of War. "But we are concerned about the upwards of 15,000 survivors around the country that are still in need of support."

The $90 million to clean up bombs joins another $100 million the US has committed in the past 20 years. The Lao government, meanwhile, says it will boost efforts to recover remains and account for Americans missing since the war.

The punishing air campaign on Laos was an effort to cut off communist forces in neighbouring Vietnam. American warplanes dropped more explosives on this Southeast Asian nation than on Germany and Japan combined in World War II, a stunning statistic that Obama noted during his first day in Vientiane.

Obama was one of several world leaders visiting Laos to attend the Association of Southeast Asian Nations. Taking its turn as chair of the regional forum, Laos' communist government is seizing a rare moment in the spotlight.

For Obama, the visit serves as a capstone to his yearslong effort to bolster relations with Southeast Asian countries long overlooked by the United States. The outreach is a core element of his attempt to shift US diplomatic and military resources away from the Middle East and into Asia in order to counter China in the region and ensure a US foothold in growing markets.

Yet Obama's outreach took an uncomfortable turn just as he headed to Laos from another summit in China. The White House called off a scheduled meeting Tuesday with President Rodrigo Duterte of the Philippine — a US treaty ally — after the brash new leader referred to Obama as a "son of a bitch."

Duterte, who had been expecting Obama to criticise his deadly, extrajudicial crackdown on drug dealers, later said he regretted the personal attack on the president.

Obama filled the hole in his schedule by meeting with South Korean President Park Geun-hye in a display of unity a day after North Korea fired three ballistic missiles. Obama vowed to work with the United Nations to tighten sanctions against Pyongyang, but said the door wasn't closed to a more functional relationship.

Obama's Asia project — dubbed his pivot or rebalance — has yielded uneven results, as conflict in the Middle East has continued to demand attention and China has bristled at what it views as meddling in its backyard.

So with just four months left in office, Obama used his historic trip to Laos to reassert his aims. He touted new military aid and US support for regional cooperation in addressing maritime disputes and made a plug for the massive Trans-Pacific Partnership free trade agreement, the policy's central economic component that is now stuck in Congress.