Laos wants to see maritime rights respected and avoid a military build-up in the South China Sea, U.S. Secretary of State John Kerry said on Monday, after a meeting with Laos' Prime Minister Thongsing Thammavong to urge ASEAN unity in the face of Chinese claims.

Laos is the 2016 chair of the 10-nation Association of Southeast Asian Nations (ASEAN) and hosts a summit later in the year that will include the leaders of the United States and China.

"He was very clear he wants a unified ASEAN and he wants maritime rights protected, and he wants to avoid militarization and to avoid conflict," Kerry told reporters after meeting Thongsing in Vientiane, the capital of Laos.

Kerry, only the third U.S. secretary of state to visit Cambodia since John Foster Dulles in 1955 and Hillary Clinton in 2012, was responding to a question whether Laos would take a strong line on territorial disputes in the South China Sea as ASEAN chair.

Laos has close political and economic ties with giant neighbor China, prompting the Obama administration to worry that Vientiane might behave like Cambodia did when it held the ASEAN chair in 2012.

Cambodia was accused of obstructing consensus in the bloc over standing up to China's assertive pursuit of its South China Sea claims, which have since included the building of artificial islands suitable for military use.

"It is particularly important that Laos finds itself playing a critical role within ASEAN, and ASEAN itself is critical to upholding the rules-based system in the Asia-Pacific and ensuring that every country, big and small, has a say in addressing the matters of shared concern," Kerry said.

"We want everybody to have a voice within the region without regards to size, power and clout."

Kerry will travel on to Cambodia later on Monday as part of his effort to urge ASEAN unity ahead of a summit President Barack Obama has called with leaders of the bloc for Feb. 15-16 in Sunnylands, California.

Kerry heads to Beijing for talks on Wednesday with the leadership there.

A senior U.S. State Department official said Kerry would seek to set an encouraging tone in Laos by discussing increased U.S. aid, including more funds for work to dispose of unexploded U.S. ordnance left over from the Vietnam War, when Laos became one of history's most heavily bombed countries, as the United States tried to destroy communist supply lines running through it.

Kerry, who fought in the Vietnam War and then became a champion of post-war reconciliation, said the United States had boosted funding for the disposal of unexploded ordnance over the years "and we are looking at whether or not that could be plussed-up even more."

The number of people killed or badly hurt by such ordnance had fallen to about 50 a year, from about 300 a few years ago, he said, adding, "Fifty a year is still too many."

The senior U.S. official said announcements on additional funding could be expected when Barack Obama becomes the first U.S. president ever to visit the country when he attends an ASEAN summit in Laos in September.

After meeting the prime minister, Kerry visited That Luang Stupa, the most important Buddhist monument in Laos, and offered a bouquet of closed white lotus blossoms dedicated to its people.

In Cambodia, Kerry will meet Hun Sen, now Asia's longest serving prime minister, and will draw attention to U.S. concerns about human rights and the treatment of government critics by meeting opposition members and civil activists, the State Department official said.

Ties between the former Cold War foes have warmed over the last 15 years or so, following cooperation over efforts to locate American soldiers missing during the Vietnam war and to dispose of unexploded U.S. ordnance.