PHNOM PENH (AFP) - America's top diplomat, Mr John Kerry, will meet Cambodian leader Hun Sen on Tuesday (Jan 26), with concerns over political repression and human rights topping the agenda as the kingdom's strongman looks to extend a three-decade stranglehold on power.

Mr Kerry, who arrived in Phnom Penh from Laos late on Monday, will also discuss deepening economic ties with Cambodia, which has emerged from the ashes of Khmer Rouge rule into one of South-east Asia's fastest-growing economies.

But Cambodia's fragile democracy appears under threat with Mr Hun Sen, who has ruled for 31 years through a mix of hard power and political guile, tying up a resurgent opposition in legal charges.

The opposition Cambodian National Rescue Party made big gains in 2013 elections, but has since seen its chief Sam Rainsy go into self-exile to avoid arrest over charges he says are politically motivated.

In a show of support, Mr Kerry will meet the opposition's deputy leader.

"The relationship between the ruling party and the opposition party is fraught right now," a State Department official told reporters.

"The leader of the opposition party is out of the country, so Secretary Kerry will meet with not only the government but also the opposition."

He will also meet civil society groups, the official added, to reinforce America's backing "for human rights, for civil rights, and for political space".

Campaigners have called on the US to exert greater pressure on Mr 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 ramping up curbs on criticism, several people have been arrested in recent weeks over Facebook posts insulting or lampooning him and his family.

Mr Kerry's trip to South-east Asia paves the way for a summit hosted next month by President Barack Obama in California with the 10 leaders of the Association of South-east Asian Nations (Asean).