U.S. defense officials say the United States is planning to expand its missile defenses in Asia, in response to threats from North Korea and aggressive moves by China.

News reports Thursday quoted officials as saying the buildup could include a new radar in southern Japan and possibly another one in Southeast Asia. The X-band radars would be linked to missile defense ships and land-based interceptors.

The Wall Street Journal said the plan is part of the Obama administration's new defense strategy to shift resources to an Asian-Pacific region critical to the U.S. economy after a decade of war in Iraq and Afghanistan.

A spokeswoman for the State Department, Victoria Nuland, said Thursday that the United States is taking a phased approach to missile defense in Asia, as it is in Europe and in the Middle East. She emphasized that those are defense systems and will not be used unless “missiles have been fired.” But she did not comment on any specific plans.

China did not comment on the reports directly, but its defense ministry issued a statement Thursday saying that “China has always believed that anti-missile issues should be handled with great discretion, from the perspective of protecting global strategic stability and promoting strategic mutual trust among all countries.”

The United States and its allies in the region have expressed growing concern about North Korea's missile tests and nuclear arms programs. Despite warnings by the international community, Pyongyang fired a rocket in April, saying it was meant to launch an experimental satellite. But it is generally believed it was a missile test.

China has angered its neighbors with aggressive moves in a maritime area claimed by several governments, including Japan, the Philippines, Taiwan and others. China also has been boosting its military strength in recent years.