PHNOM PENH (Reuters) - 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. Trials of senior political figures in recent years have resulted in convictions and international rights groups questioned whether proceedings against Kem Sokha would be fair. He 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.  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 doubts 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. Next year s election could represent the greatest challenge to Hun Sen and his ruling Cambodian People s Party (CPP) in the more than three decades he has ruled, but his critics accuse him of trying to shut down all opposition in advance. Pro-government website Fresh News said there could be further arrests of officials from the opposition party in the case.  The government and the ruling CPP have manufactured these treason charges against Kem Sokha for political purposes, aiming to try and knock the political opposition out of the ring before the 2018 electoral contest even begins,  said Phil Robertson, deputy Asia director of New York-based Human Rights Watch. Kingsley Abott, senior international legal adviser for Southeast Asia at the Geneva-based International Committee of Jurists, said the allegations against Kem Sokha had the hallmarks of being politically motivated.  The absence of an independent and impartial prosecution and judiciary makes the delivery of a fair trial impossible in most political cases,  he said. Kem Sokha, was sentenced to five months in jail last September after failing to appear in court in connection with a case against two of his party colleagues, but he was later pardoned at Hun Sen s request. He had avoided prison on that occasion by taking refuge at his party headquarters. Kem Sokha s predecessor as party leader, Sam Rainsy, was found guilty of defamation in absentia. He lives in France to avoid the conviction, which he says was politically motivated. 