US Secretary of State John Kerry on Monday hinted Washington may boost the $15 million fund it provides to tackle the deadly legacy of unexploded American bombs in Laos, during a rare visit to the reclusive communist state.

The trip paves the way for a summit hosted next month by President Barack Obama in California with the ten leaders of the Association of Southeast Asian Nations (ASEAN).

Laos has assumed this year's chairmanship of the regional bloc and will see a flurry of diplomatic activity culminating later in the year in a visit by Obama -- the first by a sitting US leader to the resource-rich but impoverished nation.

Kerry's visit is only the third since 1955 by a US Secretary of State to a country carpet-bombed by America during the Vietnam War.

Unexploded bombs across the region are the result of the massive US bombing campaign aimed at disrupting North Vietnamese supply routes through landlocked Laos.

That grim legacy carries a particular resonance for Kerry, a decorated Vietnam war veteran wounded during combat, who said the US is "deeply engaged in trying to de-mine and deal with the unexploded ordnance issue."

Laos was the most bombed nation in the world per capita, with more than two million tonnes of explosive dumped on the country.

Around 30 percent of the bombs failed to explode, including cluster munitions.

Around 50,000 people have been killed by leftover ordnance since the end of the war, with tens of thousands of others maimed, including children.

The US has gradually lifted its funding to tackle the scourge from $5 million to $15 million this year, Kerry said.

"I know that we're looking at whether or not that could be plussed up even more," he said, suggesting Obama may bring a new pledge of funds when he visits.

-- Laos changing too --

After talks with Prime Minister Thongsing Thammavong, Kerry also hailed improving ties between the two nations after decades of tension.

"While we don't agree on everything, obviously... we also do agree on a lot of things and on the way the world is changing. And it's changing here too," Kerry said of the nation which has been run by the secretive Communist Party since 1975.

Fresh from a trip to Saudi Arabia, Kerry hailed growing economic, environmental and security co-operation, as well as Laos' chairmanship of ASEAN, as the "defining" issues of a new friendship.

The US diplomat, who is due in Cambodia later Monday, was in Laos days after the five-yearly congress of the Communist Party, which chose 78-year-old vice-president Bounnhang Vorachith as its next leader.

Relations between the US and Laos two have often been hostile, with American support for ethnic Hmong anti-communist insurgents still raw in the memory of the Laos' leadership.

Laos' poor human rights record is also a sticking point, embodied by the 2012 disappearance of prominent activist Sombath Somphone, who was last seen on CCTV footage at a police checkpoint in Vientiane.

The Obama administration has made ties with Asia a diplomatic priority, in particular bolstering ASEAN as a counterpoint to Chinese regional power.

The diplomat said he was encouraged by the Laos premier's commitment to a keeping "unified front" in the face of the big issues facing ASEAN.

"He wants maritime rights protected and he wants to avoid militarisation and avoid the conflict. And that will develop as we are going to Sunnylands," he said referencing the mid-February leaders' summit in the US.

Several ASEAN states are embroiled in an increasingly bitter spat with China over disputed territory in the South China Sea.

The US says it takes no position on ownership of the various reefs and islets under dispute, but insists freedom of navigation in the vital shipping lane must be maintained.