* Long-sought agreement to be announced Sunday in Seoul
* Source says deal extends South Korea's missile range
* Move likely to rattle North Korea
By Jim Wolf
WASHINGTON, Oct 5 (Reuters) - The United States and South Korea are set to announce this weekend a joint response to the perceived threat from North Korea's growing ballistic missile force, according to a State Department notice to U.S. lawmakers on Friday.
A senior U.S. congressional aide said he understood that the two countries have agreed to a watershed deal under which Seoul may develop missiles capable of hitting any part of the North from anywhere in the South.
The notice, obtained by Reuters, said the United States and South Korea had been discussing "counter-measures" as an alliance to the threat posed by the North's ballistic missile arsenal.
The South Korean government will make a public announcement of the outcome of the talks on Sunday, the notice said. The State Department and the South Korean embassy in Washington declined to comment.
Seoul for years has sought to push its missile range beyond 300 kilometers (186 miles), the limit under a pact with the United States, which maintains about 28,500 troops in the South.
The deal to be announced in Seoul would stretch the range to 800 kilometers (497 miles), largely in line with the South's push over recent months, according to the congressional aide who spoke on condition of anonymity.
Any such extension is bound to rattle North Korea, which has remained at odds with the South since the 1950-53 Korean War left the peninsula divided.
It also likely would stir Japan and China, parts of which would be within range of 800-km South Korean missiles, said Greg Thielmann, who formerly took part in intelligence assessments on ballistic missile threats at the State Department's intelligence bureau.
Steven Hildreth, a missile expert at the nonpartisan Congressional Research Service, said North Korea, which from time to time has threatened to attack Seoul, "could easily use this announcement for whatever suits their threat du jour."
Washington had long sought to discourage South Korea from developing longer-range ballistic missiles in keeping with a voluntary international arms-control pact known as the Missile Technology Control Regime (MTCR).
The pact aims to curb the spread of ballistic missiles and other delivery systems that could be used for chemical, biological and nuclear attacks.
It urges its 34 members, including most major missile manufacturers, to restrict their exports of missiles and related technologies capable of carrying a 500-kilogram (1,102-pound) payload at least 300 kilometers or delivering any type of weapon of mass destruction.
Arms-control advocates say a U.S. decision to help South Korea boost its missile range beyond the MTCR limits would undermine the pact and open the flood gates to missile proliferation, notably by others who may feel less constrained.
"Agreeing for any country to develop 800-km range missiles, well outside the MTCR limits, would be a big mistake," said Thielmann, now of the private Arms Control Association in Washington.
Since its inception in 1987, the agreement has been credited with slowing or stopping several missile programs.
According to South Korean defense officials, Pyongyang's arsenal includes intermediate-range missiles that have a range of 3,000 kilometers (1,520 miles), which includes all of Japan and the U.S. military bases located there. The North is not a member of the MTCR.
(Editing by Paul Simao)
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Keywords: USA KOREA/MISSILES