AP Explains: Why Asian diplomacy is on nuke summit sidelines

The U.S. has deepened ties with India, and strengthened long-standing alliances with Japan and South Korea while prodding these two core East Asian allies to overcome their historical differences.

Xi will want the U.S. to restart negotiations with the North, although the prospects appear slim unless sanctions bite and the North makes nuclear concessions.

Since annual U.S.-South Korean war games began this month, Pyongyang has been testing missiles and artillery, and has threatened nuclear strikes on Washington and Seoul.

Tensions there look set to intensify with an upcoming ruling from an international tribunal that could challenge the legal basis of Beijing's sweeping territorial claims.

Indian Prime Minister Narendra Modi will also be at the nuclear summit, but there's no prospect for reconciliatory talks with archrival Pakistan because its prime minister, Nawaz Sharif, canceled his Washington trip following a suicide bombing targeting Christians in Lahore that killed more than 70 people.