China's new underground nuclear submarine base close to vital sea lanes in Southeast Asia has raised US concerns, with experts calling for a shoring up of alliances in the region to check Beijing's growing military clout.  The base's existence on the southern tip of Hainan Island was confirmed for the first time by high resolution satellite images, according to Jane's Intelligence Review, a respected defence periodical, this month. It could hold up to 20 submarines, including a new type of nuclear ballistic missile submarine, and future Chinese aircraft carrier battle groups, posing a challenge to longstanding US military dominance in Asia. China should not pursue such "high-end military options," warned Admiral Timothy Keating, the top commander of US forces in Asia, in an interview with the Voice of America last week.