By Andrea Shalal and David Brunnstrom
WASHINGTON (Reuters) - North Korea's latest rocket launch might kick off a buildup of U.S. missile defense systems in Asia, U.S. officials and missile defense experts said, something that could further strain U.S.-China ties and also hurt relations between Beijing and Seoul.
North Korea says it put a satellite into orbit on Sunday, but the United States and its allies see the launch as cover for Pyongyang's development of ballistic missile technology that could be used to deliver a nuclear weapon.
Washington sought to reassure its allies South Korea and Japan of its commitment to their defense after the launch, which followed a North Korean nuclear test on Jan. 6.
The United States and South Korea said they would begin formal talks about deploying the sophisticated Terminal High Altitude Area Defense system, or THAAD, to the Korean peninsula "at the earliest possible date."
South Korea had been reluctant to publicly discuss the possibility due to worries about upsetting China, its biggest trading partner.
Beijing, at odds with the United States over Washington's reaction to its building of artificial islands in the disputed South China Sea, quickly expressed "deep concern" about a system whose radar could penetrate Chinese territory.
China had made its position clear to Seoul and Washington, the Foreign Ministry said.
"When pursuing its own security, one country should not impair others' security interests," spokeswoman Hua Chunying said in a statement.
But the North Korean rocket launch, on top of last month's nuclear test, could be a "tipping point" for South Korea and win over parts of Seoul's political establishment that remain wary of such a move, a U.S. official said.
South Korea and the United States said that if THAAD was deployed to South Korea, it would be focused only on North Korea.
An editorial in the Global Times, an influential tabloid published by the ruling Chinese Communist Party's official People's Daily newspaper, called that assurance "feeble".
"It is widely believed by military experts that once THAAD is installed, Chinese missiles will be included as its target of surveillance, which will jeopardize Chinese national security," it said.
Japan, long concerned about North Korea's ballistic missile program, has previously said it was considering THAAD to beef up its defenses. The North Korean rocket on Sunday flew over Japan's southern Okinawa prefecture.
Chief cabinet secretary Yoshihide Suga told reporters on Monday the Defense Ministry had no concrete plan to introduce THAAD, but added the ministry believed new military assets would strengthen the country's capabilities.
Riki Ellison, founder of the Missile Defense Advocacy Alliance, said the launch would give Japan momentum to deploy THAAD.
Washington moved one of its five THAAD systems to Guam in 2013 following North Korean threats, and is now studying the possibility of converting a Hawaii test site for a land-based version of the shipboard Aegis missile defense system into a combat-ready facility.
Some experts questioned how effective THAAD would be against the type of long-range rocket launched by North Korea and the Pentagon concedes it has yet to be tested against such a device.
THAAD is designed to intercept and destroy ballistic missiles inside or just outside the atmosphere during their final, or terminal, phase of flight. It has so far proven effective against short- and medium-range ballistic missiles.
John Schilling, a contributor to the Washington-based 38 North project that monitors North Korea, said THAAD's advanced AN/TPY-2 tracking radar built by Raytheon Co could provide an early, precise track on any such missile.
David Wright, co-director of the Global Security Program at the U.S.-based Union of Concerned Scientists, said that while THAAD could not shoot down the type of rocket launched on Sunday its deployment could reassure the South Korean public.
"Much of what missile defense programs are about is reassuring allies and the public," he said.
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One U.S. official said the North Korean launch added urgency to longstanding informal discussions about a possible THAAD deployment to South Korea. "Speed is the priority," said the official, who asked not to be named ahead of a formal decision.
Renewed missile-defense discussions with the United States could also send a message to Beijing that it needs to do more to rein in North Korea's ballistic missile and nuclear weapons programs, another U.S. official said.
South Korean officials have already identified a suitable site for the system, but it could also be placed at a U.S. base on the Korean peninsula, Ellison said.
THAAD is a system built by Lockheed Martin Corp that can be transported by air, sea or land. The Pentagon has ordered two more batteries from Lockheed.
One of the four THAAD batteries based at Fort Bliss, Texas, is always ready for deployment overseas, and could be sent to Japan or South Korea within weeks, Ellison said.
Lockheed referred all questions about a possible THAAD deployment to the U.S. military.
(Reporting by Andrea Shalal, David Brunnstrom and Matt Spetalnick in Washington. Additional reporting by Kiyoshi Takenaka and Tim Kelly in Tokyo and Ben Blanchard in Taipei; Editing by Dean Yates and Lincoln Feast)