Phnom Penh, Cambodia: US Secretary of State John Kerry met Cambodian leader Hun Sen Tuesday, praising the kingdom's "remarkable" economic revival but pushing for greater political freedoms as the strongman tries to extend his three-decade grip on power.

Cambodia has emerged from the ashes of the genocidal Khmer Rouge regime to become one of Southeast Asia's fastest-growing economies.

But it provides political support to China within the Association of Southeast Asian Nations, an alliance that has at times caused strain within the bloc in the face of increasingly aggressive Chinese claims in the South China Sea.

President Barack Obama has assiduously courted ASEAN as part of a diplomatic 'pivot' east aiming to offset China's trade and diplomatic might in Southeast Asia.

Kerry, who arrived in Phnom Penh from neighbouring Laos, praised Cambodia for rebooting its economy after the fanatical rule of the Maoist Khmer Rouge, which left up to a quarter of the population dead before its defeat in the mid-1970s.

Although some three million people still live below the poverty line, the World Bank said the economy is on track to expand just under seven percent this year.

Hailing the "remarkable growth", Kerry said Cambodia was "about the cross the line" into becoming a middle-income country. 

"You have seen incredible changes," he said ahead of talks with Hun Sen.

Garment manufacture is a key pillar of that success, with an industry lobby group saying exports to the US alone were worth $1.8 billion in 2014.

- Fraught politics -

But economic gains are being offset by the threat posed to Cambodia's fragile democracy. 

Hun Sen, who has ruled for 31 years through a mix of hard power and political guile, smothered a resurgent opposition in legal charges over recent months.

As a result the leader of the Cambodian National Rescue Party is now in self-exile, casting doubt over whether the party will be able to freely contest the next election slated for 2018.

"The relationship between the ruling party and the opposition party is fraught right now," a State Department official briefed reporters ahead of Kerry's meeting with opposition deputy leader Kem Sokha.

After the closed-door session Kem Sohka said he told Kerry his party wants free and fair elections.

"We raised the issue of the democratic process... electoral work, and its climate," he added.

The UN has warned that ongoing political instability could destabilise the country.

Kerry was also set meet civil society groups, the US official added, to reinforce America's support "for human rights, for civil rights, and for political space".

Campaigners have called on the US to exert greater pressure on Hun Sen to end repression, including of political opponents, dissenters and trade unionists representing low-paid workers.

But in a sign the prime minister is tightening controls on criticism, several people have been arrested in recent weeks over Facebook posts insulting or lampooning him and his family.

Kerry's trip to Southeast Asia paves the way for a summit to be hosted by Obama in California next month with the 10 ASEAN leaders.

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

Kerry is due to fly to Beijing on Tuesday afternoon.