PHNOM PENH (Reuters) - Cambodia s main opposition leader Kem Sokha was arrested and accused of treason on Sunday and a leading independent paper said it was being forced to shut down as Prime Minister Hun Sen s government intensified a crackdown on his critics. Hun Sen said Kem Sokha had been plotting with the United States, increasing his anti-American rhetoric as Cambodia heads towards elections next year in which the authoritarian leader s grip of more than three decades is at stake. The U.S. State Department expressed concern at the arrest of Ken Sokha and action against the media. It raised doubt over whether the Southeast Asian country can hold a fair election. Hun Sen, 65, a former Khmer Rouge cadre, who is now one of China s closest regional allies, said Kem Sokha had been arrested for treason in which the United States was implicated.  It s an act of treason with conspiracy with a foreign country, betraying his own nation. This requires arrest,  Hun Sen told a group of garment workers, according to the pro-government Fresh News website. Kem Sokha, 64, has led the main opposition Cambodia National Rescue Party (CNRP) since his predecessor resigned in February, saying he feared a government plan to shut it down. Kem Sokha was taken from his home in handcuffs after a night-time police raid. The opposition party lost to Hun Sen s Cambodian People s Party in local elections in June, but it did well enough to increase expectations of a close contest in the 2018 vote. The government released a video on its Facebook page in which Kem Sokha appeared to tell a group of supporters about a strategy to win power which he said had the support of  the Americans  rather than an immediate plot to topple Hun Sen. In the video, Kem Sokha said the Americans had hired academics to advise on strategy to change Cambodia s leaders.  And if I follow such a tactic and strategy and if I could not win, I do not know what else to do,  he said. The opposition party made no immediate comment on the veracity or content of the video. Earlier, it said Kem Sokha s arrest was politically motivated and violated the law because of the immunity granted to elected lawmakers. The party called for his release and urged the international community to intervene. If Kem Sokha is found guilty of any offence, it could allow the government to shut the party under a new law. Kem Sokha made no immediate comment and it was not clear if he had been charged or had legal representation at this stage. The government has stepped up attacks on the United States and last month ordered the expulsion of the U.S. State Department-funded National Democratic Institute pro-democracy group. Earlier in the year, it suspended joint military exercises with the United States, which has voiced fears over the human rights situation. State Department spokeswoman Heather Nauert said the charges against Kem Sokha appeared to be politically motivated. Her statement did not directly address Hun Sen s accusations against the United States. Nauert also highlighted recent steps by the Cambodian government against independent media and civil society, saying they  raise serious questions about the government s ability to organize credible national elections in 2018 which produce an outcome that enjoys democratic legitimacy . In a sign of the pressure on the media, The Cambodia Daily said it was ceasing publication after the government ordered it to pay a crippling $6.3 million tax bill by Monday. The English-language paper, founded by an American journalist, was known for critical coverage of issues such as corruption, human rights and the environment.  After 24 years and 15 days, the Cambodian government has destroyed The Cambodia Daily, a special and singular part of Cambodia s free press,  it said in a statement. During Hun Sen s rule Cambodia emerged from the devastating Khmer Rouge genocide to enjoy record years of economic growth of  above 7 percent, but disaffection has been increasing and he only just won the 2013 election against a unified opposition. 