A Patriot rocket interceptor unit based in Okinawa has arrived in South Korea for use in a military drill, Japanese and Korean news agencies said Friday. It is the first time a Japan-based anti-missile unit has been sent to South Korea.

The PAC-3 battery was transported to a U.S. Air Force base in Gunsan, South Korea, in the light of a slew of missile launches by North Korea, Kyodo News quoted a U.S. military official in South Korea as saying.

"The drill is aimed at rapidly transporting the U.S. anti-missile unit from Japan to South Korea by ship in case of a conflict on the Korean Peninsula," the Yonhap news agency quoted a U.S. Forces Korea official as saying. "After the two-week joint military exercises, the PAC-3 unit will be shipped back to Japan."

The Patriot unit comprises 120 troops with a launch vehicle and radar, Kyodo said. It was shipped from Okinawa to the coastal city of Busan, South Korea, and then to Gunsam.

Seoul and Washington said this month they will deploy an advanced high-altitude anti-missile battery in South Korea by the end of next year. A site at Seongju in the south of the country has been earmarked for the rockets, which South Korean officials say will provide robust defense of targets such as cities, nuclear power plants and military bases.

The planned deployment has riled North Korea and drawn criticism from China and Russia, who say it will upset the balance of power in the region.

On Tuesday, North Korea fired three missiles, which South Korea's military identified Friday as two mid-range Rodong rockets and one short-range Scud. All are potentially nuclear-capable.

The military said one of the three exploded prematurely.