Cambodian opposition leader Kem Sokha has been charged with treason and could face a jail term of 15 to 30 years if convicted. Picture: AP Photo/Heng Sinith, File

Phnom Penh - Cambodian opposition leader Kem Sokha has been charged with treason and could face a jail term of 15 to 30 years if convicted, a court said on Tuesday.

Kem Sokha was arrested on Sunday in an escalating crackdown on critics of Prime Minister Hun Sen's government, which accused the opposition leader of plotting with the United States to undermine the Southeast Asian country.

Kem Sokha had been charged with "colluding with foreigners" under Article 443 of Cambodia's penal code, the Phnom Penh Municipal Court said in a statement.

"The act of secret collusion with foreigners is an act of treason," it added.

The evidence the government has presented is a video of Kem Sokha from 2013 in which he tells supporters of his Cambodia National Rescue Party (CNRP) that he has had American support and advice for his political strategy to win power.

One of the opposition leader's lawyers, Pheng Heng, said what appeared in the video clip was no crime.

"The legal procedure is wrong and the charge isn't correct," he said. "His words are educational in a workshop ... What he talked about was elections in a multi-party democratic way."

The arrest of Kem Sokha and growing pressure on independent media and rights groups have drawn condemnation from the United States and other Western countries, which have raised questions over whether a general election next year can be fair.

But Hun Sen, one of Asia's longest serving rulers, has won support from China, which has made him one of its closest regional allies and provided billions of dollars in infrastructure loans.

The election could represent Hun Sen's greatest electoral challenge in more than three decades in power, but his opponents accuse him of trying to shut down all opposition in advance.

Lawyers met Kem Sokha on Monday for 20 minutes and said his health was fine.

"I may lose freedom, but may freedom never die in Cambodia," Kem Sokha was quoted as saying in a post on Twitter that was repeated by his daughter, Monovithya Kem.

The European Union has called for his immediate release, based on the fact that he is meant to have parliamentary immunity, as an elected lawmaker.

The U.S. State Department expressed "grave concern" at his arrest on charges it said appeared to be politically motivated.

U.N. High Commissioner for Human Rights Zeid Ra’ad al-Hussein said on Monday he was seriously concerned about the arrest and the evidence against the opposition leader.