WASHINGTON—The Biden administration will seek to bolster economic and security cooperation with Southeast Asia through a visit to the region next week by its top diplomat, as it works to forge a united front against China in the Indo-Pacific. Secretary of State Antony Blinken is due in the Indonesian capital Jakarta on Monday and will also visit Malaysia and Thailand on his first Southeast Asia trip since President Joe Biden took office in January. Southeast Asia has become a strategic battleground between the United States and China, the world’s two largest economies. China claims most of the South China Sea, the vital trade route that links the region, and has turned up military and political pressure on self-ruled Taiwan, which Beijing considers its own. Blinken will pursue Biden’s aim of elevating engagement with the Association of Southeast Asian Nations (ASEAN) to “unprecedented” levels, focusing on strengthening regional security infrastructure in …