A prominent Cambodian opposition senator who angered Prime Minister Hun Sen with a Facebook posting of a phony pledge to dissolve the border with Vietnam has been arrested and will be charged with treason, a government spokesman said.Senator Hong Sok Hour of the opposition Cambodia National Rescue Party (CNRP), who posted a fake border treaty with Vietnam, has been "detained in the police department for questioning", spokesman Phay Siphan said.The party of Sok Hour, who is also a French citizen, recently alleged Vietnamese encroachment into Cambodia. The online posting said the two countries pledged to dissolve their border.On Thursday, Hun Sen ordered the arrest of Sok Hour, who went into hiding."The offense is a treason," Hun Sen said. "Please, embassies, don't allow him in if he runs into," he added.A CNRP lawmaker from the lower house, Ou Chanrith, said the posted treaty is a fake, but said Sok Hour had not committed a treason and his arrest was "illegal" as senators have immunity from arrest or prosecution.Phay Siphan, the government spokesman, said the senator will be charged in court with treason. The senator's lawyer said he did not know what the possible punishment would be if there is a conviction for treason.Tension has been rising between CNRP and the ruling Cambodian People's Party, which struck a political deal last year to foster a "new culture of dialogue".