By Soo-hyang Choi and Hyonhee Shin
SEOUL/BUSAN, South Korea (Reuters) - North Korea fired two short-range ballistic missiles off its east coast on Monday, South Korea's military said, just as a U.S. aircraft carrier staged combined naval exercises with South Korea in a warning to Pyongyang.
The missiles flew about 370 km (230 miles) after being launched from North Hwanghae province at 7:47 a.m. (2247 GMT on Sunday), South Korea's Joint Chiefs of Staff (JCS) said.
The North's latest launch came as a U.S. carrier strike group led by USS Nimitz joined military exercises with South Korea in international waters off the southern island of Jeju.
The carrier is scheduled to arrive at a naval base in Busan on Tuesday in its first docking in South Korea for nearly six years, partly to mark the 70th anniversary of the alliance, Seoul's defence ministry said. In September, another aircraft carrier USS Ronald Reagan visited the port.
The drills were designed to improve the execution of U.S. extended deterrence - its military capability, especially nuclear forces, to deter attacks on its allies - by deploying American strategic assets amid the North's growing threats, a South Korean navy official said.
Rear Admiral Christopher Sweeney, strike group commander, did not specify how it detects and responds to North Korean missile tests. But he said it was "well informed" about such activities, serving as a command and control centre gathering information across all domains from space to under the sea.
"It's important for us to be able to integrate with our navy allies and share information and our interoperability, because we don't like to be coerced - I don't think anyone likes a bully," Sweeney told reporters from the carrier.
When asked about growing calls in South Korea for permanently deploying American strategic assets, he said: "The United States has deployable strategic assets at the ready on every day and we can continue to deploy those assets."
GRAPHIC: North Korean ballistic missile tests (https://www.reuters.com/graphics/NORTHKOREA-MISSILES/zgpobnqyjvd/chart.png)
North Korea has been ramping up its military tests in recent weeks, firing multiple cruise missiles to practice tactical nuclear attacks, and testing what it called a nuclear-capable underwater attack drone.
South Korea's military condemned the North's launches as a grave provocation and said it would continue building the capability to "overwhelmingly respond to any provocations".
The U.S. Indo-Pacific Command said the latest launch did not pose an immediate threat, but highlighted the destabilising impact of Pyongyang's unlawful nuclear weapons programmes.
The Japanese government also said it lodged a "strong protest" with North Korea.
Japanese Chief Cabinet Secretary Hirokazu Matsuno said North Korea was likely to step up provocative activities, including a possible nuclear test.
U.S. and South Korean officials have for nearly a year warned that the North could carry out what would be its first nuclear test since 2017.
The allies have been conducting a series of joint training in recent weeks including air and sea drills involving American B-1B bombers, and their first large-scale amphibious landing exercises in five years.
They concluded their regular springtime exercises, called Freedom Shield 23, last week, but have other field training continuing, also including amphibious landing drills involving a U.S. amphibious assault ship.
Pyongyang has long bristled at the allies' drills, saying they are preparation for an invasion. South Korea and the United States say the exercises are defensive.
The United States has about 28,500 troops in South Korea.