In May 1955, Sihanouk had accepted military aid from the US. The following January, when he was in the Philippines on a state visit, Central Intelligence Agency (CIA) operatives attempted to sway him into placing Cambodia under Southeast Asia Treaty Organization (SEATO) protection. Subsequently, Sihanouk began to suspect that the US was attempting to undermine his government and that it was lending covert support to the Democratic party, now without parliamentary representation, for that purpose. On the other hand, Sihanouk developed a good impression of China, whose premier, Zhou Enlai, gave him a warm reception on his first visit there in February 1956. They signed a friendship treaty in which China promised in economic aid to Cambodia. When Sihanouk returned from China, Sarit Thanarat and Ngo Dinh Diem, leaders of Thailand and South Vietnam, respectively, both with pro-American sympathies, started to accuse him of pro-Communist sympathies. South Vietnam briefly imposed a trade embargo on Cambodia, preventing trading ships from travelling up the Mekong river to Phnom Penh. While Sihanouk professed that he was pursuing a policy of neutrality, Sarit and Diem remained distrustful of him, more so after he established formal diplomatic relations with China in 1958.