BEIJING (Yonhap) – U.S. Secretary of State John Kerry  is in Beijing on Saturday for talks with Chinese leaders as China's massive land reclamation efforts in the South China Sea have made its Asian neighbors nervous.

China claims sovereignty over nearly 90 percent of the South China Sea, worsening relations with neighbors such as the Philippines, Malaysia and Vietnam. Beijing is in a bitter dispute with Tokyo over islets in the East China Sea.

The U.S. has been concerned about China's increasingly assertive moves in the South China Sea, which is resource-rich and includes key shipping lanes.

Daniel Russel, the top U.S. diplomat for East Asia, told the U.S. Congress earlier this week, "Despite our differences over the South China Sea, the United States and China have worked hard to expand cooperation and develop effective channels of communication to manage differences."

However, Russel said, "We also underscore that the United States will not hesitate to defend our national security interests and to honor our commitments to allies and partners in the Asia-Pacific."

North Korea is also expected to be one of the agenda items for Kerry's talks in Beijing, ahead of his trip to South Korea on Sunday.

North Korea has claimed that it successfully test-launched a ballistic missile from a submarine. If confirmed, it would pose a significant security threat to South Korea.

Also this week, South Korea's intelligence agency told lawmakers that the North's young leader, Kim Jong-un, ordered his defense chief executed for showing disloyalty.

The moves by North Korea came as five nations involved in long-stalled talks, including the U.S. and China, on the North's nuclear weapons program, agreed on the need to hold "exploratory talks" with Pyongyang without any preconditions to test the reclusive nation's denuclearization commitment before resuming formal negotiations.

The six-party talks aimed at persuading North Korea to give up its nuclear ambition, involving South Korea, North Korea, the United States, China, Japan and Russia, have been dormant since late 2008.

Since then, North Korea has advanced its nuclear capabilities by conducting its second and third nuclear tests, in 2009 and 2013.

Some experts now warn that the communist nation's nuclear arsenal could expand to 100 bombs by 2020.