Hillary Clinton Wednesday became the first US secretary of state to visit Laos for 57 years, on a trip focused on the damaging legacy of the Vietnam War and a controversial dam project.

At Vientiane's flag-bedecked Wattay International airport, Clinton was given flowers by girls in traditional purple-silk costumes, kicking off her brief but historic trip.

The US "desire was to deepen and broaden" the relationship, Clinton said after a visit to a US-funded orthotic and prosthetic centre, which she described as a "painful reminder of the legacy of the Vietnam War era".

"Here in Laos the past is always with you," she said, addressing US embassy employees.

US forces dropped more than two million tons of ordnance on Laos between 1964 and 1973 in some 580,000 bombing missions to cut off North Vietnam supply lines.

Some 30 percent of the ordnance failed to detonate. All 17 of the country's provinces are still contaminated by unexploded ordnance and Laos remains the most heavily bombed country, per capita, in history.

Clinton, making a four-hour whirlwind trip, met Prime Minister Thongsing Thammavong at his office in an elegant white-columned building with two large elephant statues outside.

The pair had "substantive discussions on the broadening bilateral cooperation", according to a joint statement released after the meeting.

The countries "agreed to improve and further facilitate the accounting operations for American personnel still missing from the Indochina War era" and address the "remaining challenges" of unexploded ordnance, the statement said.

They also discussed the forthcoming entry of landlocked Laos, one of the poorest nations in the world with just 6.5 million people and annual GDP growth of seven percent, into the World Trade Organization (WTO).

Clinton also held talks with Foreign Minister Thongloun Sisoulith and she said they had "traced the arc of our relationship from addressing the tragic legacies of the past to finding a way to be partners in the future".

Clinton is only the second secretary of state to visit Laos after John Foster Dulles, who spent a day in the then-monarchy in 1955.

She was invited to Laos by Sisoulith in 2010 who was the first top Laotian official to visit Washington since the Soviet-backed communist rebels swept to power, ousting the monarchy, in 1975.

US relations with Laos, while never severed, were long tense, in part over its campaign against the Hmong hill people who assisted US forces during the Vietnam War.

But the United States established normal trade ties with Laos in 2004 and annual US aid to Laos will be around $30 million in total for 2012, a US embassy official said.

Of that, $9.2 million will be set aside this year for cleaning up unexploded ordnance. According to official figures, there has been a fall in accidents involving such munitions from 300 a year to roughly 92 in 2011.

Clinton said she hoped in the future there would by ways "to give people and particularly children of this nation the opportunity to live their lives free of these unexploded bombs".

Another of the main thrusts of the trip was talks on controversial plans by Laos to build a massive dam on the Mekong River, which governments and environmentalists warn could have a devastating effect on millions of people.

During regional talks in Bali last year, Clinton called for a moratorium on dam building along the river -- the world's largest inland fishery. Activists say the dam projects could spell disaster for 60 million people who depend on the Mekong waterway.

A senior state department official said the US welcomed a recent move by Laos to stall the construction of the $3.8 billion hydroelectric project at Xayaburi until its neighbours' environmental concerns have been answered.