South Korea's military has said that it had successfully retrieved a large chunk of a crashed North Korean space rocket from the sea bed after 15 days of complex salvage operations.

Friday's announcement came as Seoul welcomed the nuclear-powered American submarine USS Michigan, as part of a recent bilateral deal on enhancing “regular visibility” of US strategic assets to the Korean Peninsula in response to North Korea's recent ballistic missile launches.

North Korea attempted to put its first military spy satellite into orbit on May 31, but the projectile and its payload crashed into the sea shortly after launch due to what Pyongyang said was a rocket failure.

After deploying a fleet of naval rescue ships and minesweepers plus dozens of deep-sea divers, South Korea's military said it had managed to salvage what appeared to be the main body of the rocket late on Thursday from the Yellow Sea.

"The salvaged object is scheduled to be analysed in detail by specialised institutions such as the national agency for defense development," the Joint Chiefs of Staff said in a statement.

The wreckage was pulled from the sea bed at a depth of about 75 metres (250 feet) in waters about 200 kilometres (125 miles) southwest of Eocheong Island, it added.

Images released by Seoul's defence ministry showed a long, white barrel-like metal structure with the word "Chonma" written on it - possibly a shorter form of the rocket's official name, Chol lima-1.

The rocket was named after a mythical winged horse that often features in Pyongyang's propaganda.

The May 31 launch was slammed by the United States, South Korea and Japan, saying it violated UN resolutions barring the nuclear-armed country from any tests using ballistic missile technology.

Analysts have said there is significant technological overlap between the development of intercontinental ballistic missiles and space launch capabilities of North Korea.

Seoul has been working for the last two weeks to recover the wreckage of the space rocket, as the debris could help scientists gain insight into Pyongyang's ballistic missile and satellite surveillance programmes.

North Korea vowed after the May 31 failure that it would successfully launch its spy satellite soon.

Pyongyang has previously claimed its military spy satellite is necessary to counterbalance the growing US military presence in the region.

Relations between the two Koreas are at one of their lowest points in years, with diplomacy stalled and North Korea's leader Kim Jong-un declaring his country an "irreversible" nuclear power, as well as calling for ramped-up weapons production, including of tactical nukes.

'Growing threats'

In a separate development, the nuclear-powered submarine USS Michigan, which is capable of carrying about 150 Tomahawk missiles, arrived in Busan, South Korea on Friday, a day after North Korea resumed missile tests in protest of the US-South Korean live-fire drills.

USS Michigan’s arrival in South Korea is the first of its kind in six years

With the deployment, the US and South Korean navies are to conduct drills on boosting their special operation capabilities and joint ability to cope with growing North Korean nuclear threats, the South Korean Defense Ministry said in a statement.

The statement did not say how long it would stay in South Korean waters.

The USS Michigan is one of the biggest submarines in the world. The Ohio-class guided-missile submarine can be armed with Tomahawk missiles with a range of about 2,500 kilometers (1,550 miles) and is capable of launching special forces missions.

The South Korean and US militaries have been expanding their exercises in reaction to North Korea’s provocative run of missile tests since last year.

On Thursday, North Korea fired two short-range ballistic missiles off its east coast, shortly after it vowed responses to the just-ended South Korea-US firing drills near the Koreas’ heavily armed border.