PHNOM PENH • Cambodian opposition leader Kem Sokha was charged with treason and espionage yesterday over an alleged conspiracy with unnamed foreigners, an offence that carries up to 30 years in jail.

He is accused of conniving in a "secret plan" with foreign entities which began in 1993, according to a court statement. Evidence of a conspiracy was substantial enough to charge the Cambodia National Rescue Party (CNRP) chief under the penal code section for "treason and espionage", it added.

Kem Sokha, 64, was arrested early on Sunday in a swoop by hundreds of security force members at his home in the Cambodian capital Phnom Penh. He has formally led the CNRP since March.

Hours later, Prime Minister Hun Sen - Asia's longest-serving leader - accused the politician of being in cahoots with the United States. "Behind his (Kem Sokha's) hand it is still the same, it is America."

To back up his claims, Mr Hun Sen cited a publicly available 2013 speech that the opposition politician gave in Australia in which he said he had received US help to build a pro- democracy movement inside Cambodia.

A report on the speech also featured on the pro-government Fresh News website shortly before the arrest.

Washington has yet to address the espionage allegation but the State Department said the charges against Kem Sokha "appear to be politically motivated".

It added that his arrest followed "a number of troubling recent steps" against civil society and the free media that threatened to undermine the credibility of next year's elections.

United Nations rights chief Zeid Ra'ad Al Hussein said he was "seriously concerned" by the arrest, adding that it "appears to have been carried out with no respect for due process guarantees, including respect for Kem Sokha's parliamentary immunity".

The surprise detention of Kem Sokha, an elected lawmaker, raises the political temperature in Cambodia. The CNRP had been widely tipped to perform strongly in next year's elections, buoyed by the youth vote in a country where many are tired of rampant corruption, inequality and the dwindling respect for human rights.

In February, the CNRP's then- leader Sam Rainsy stepped down after a fresh slew of legal cases against him threatened the party with a ban from politics. He is living in France to avoid convictions in Cambodia, which he says are politically motivated.

Kem Sokha took over at the helm of the party, but has been buffeted by allegations and threats driven by Prime Minister Hun Sen.

On Monday, a re-tweet on Kem Sokha's official Twitter feed read: "I may lose my freedom, but may freedom never die in Cambodia."

Analysts say Mr Hun Sen, a former Khmer Rouge soldier, is determined to extend his three-decade rule, and likely to withstand the burgeoning popularity of the CNRP, muffling critics in the media and in civil society.

AGENCE FRANCE-PRESSE