Pentagon Chief Says U.S. Is Watching N. Korea Missile Program

North Korean leader Kim Jong Un organizes and guides a combined joint drill of the units under KPA Combined Units 572 and 630 in this undated file photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang November 23, 2014. KCNA/Files/Reuters

MARINE CORPS AIR STATION MIRAMAR, Calif. (Reuters) - U.S. Defense Secretary Ash Carter said on Wednesday the U.S. military was keeping a vigilant eye on North Korea's missile and nuclear programs, and was continually expanding its defenses against a possible missile attack by Pyongyang.

Carter said the United States was on track to expand the number of ground-based missile interceptors in Alaska and Hawaii to 44 from 30, and improve their quality, but no further interceptor expansion was planned for now.

Asked if Washington was looking to add more interceptors to those already planned, Carter told reporters: "At the moment, no. That calculus stands... The plan has not changed."

North Korea told U.N. agencies on Tuesday it plans to launch a satellite as early as next week, a move that could advance the country's long-range missile technology after its fourth nuclear test on Jan. 6.

News of the planned launch drew fresh U.S. calls for tougher U.N. sanctions already under discussion in response to North Korea's nuclear test last month.

Carter said the United States remained concerned about North Korea's development of intercontinental ballistic missiles and nuclear weapons, and was investing heavily to continue to improve U.S. defenses against a possible attack.

The defense secretary spoke with reporters after a visit to Marine Corps Air Station Miramar in California, where he vowed to increase funding for servicing aging aircraft and accelerate purchases of new planes to ensure higher readiness rates.

Carter said U.S. Marines and other military forces stood ready to "fight tonight" on the Korean peninsula, part of a strategy aimed at deterring North Korea from ever launching an attack on the United States.

He said the U.S. military was also working hard to improve the quality and reliability of the existing ground-based interceptors in the United States.

The U.S. Missile Defense Agency last week conducted a successful test of the ground-based U.S. missile defense system managed by Boeing Co <ba.n> and a redesigned "kill vehicle" or warhead built by Raytheon Co <rtn.n>.

A senior Boeing official said it would test additional improvements to the system later this year during a first test of the system's ability to intercept an intercontinental ballistic missile, or ICBM.

The United States has been developing the $41 billion weapons system to defeat the long ranges and high velocities of an ICBM like those being developed by North Korea and Iran. It previously tested and intercepted dummy missiles that simulated short and medium-range ballistic missiles.