Cambodia’s prime minister on Thursday demanded the arrest of an opposition senator, accusing him of treason for comments posted on Facebook criticising a 36-year-old border agreement with neighboring Vietnam. Hun Sen called for the arrest of Hong Sok Hour in a speech at a graduation ceremony in Phnom Penh, in the latest in a string of attacks on political opponents. The opposition Cambodia National Rescue Party has been seeking political benefit by saying Vietnam has been encroaching on Cambodian soil, and its efforts have ramped up tension at the border.

Police said Hong Sok Hour has not yet been taken into custody, while Cambodian media cited his associates as saying he is in “a safe place.” Parliament would normally need to lift the immunity from arrest normally accorded lawmakers, which Hun Sen’s ruling party has enough votes to easily do. The opposition says the courts, under Hun Sen’s influence, are pressuring them to drop the border issue by jailing their activists on insurrection charges in connection with a violent protest last year. In recent weeks, Hun Sen has used his public speeches to deliver what amounts to arrest orders, which are generally carried out quickly.