South Korea's navy has retrieved debris from the first stage of North Korea's long-range rocket, which will be analysed to determine its level of ballistic expertise, the defense ministry said Friday.

"This debris is expected to be an important piece of information in determining North Korea's rocket capability," said defense ministry spokesman Kim Min-Seok.

The section salvaged by the navy appears to be a fuel tank, inscribed with the name of the "Unha-3" rocket.

The analysis will be carried out by a team of civilian and military experts, as well as US specialists in Soviet missile technology.

The first stage of the rocket launched on Wednesday fell in the sea off the Korean peninsula, while the second splashed down east of the Philippines.

The recovered debris was found on the sea bed, some 160 kilometers (100 miles) west of the southwestern port of Gunsan, Yonhap news agency said, at a depth of around 80 meters (260 feet).

Before its last rocket launch attempt in April -- which ended in failure -- North Korea had warned both Japan and South Korea that any effort to salvage debris from the rocket would be considered an "act of war".

The warning was not repeated before Wednesday's launch.

Pyongyang said its latest launch was a purely scientific mission aimed at placing a polar-orbiting earth observation satellite in space.

Most of the world saw it as a disguised ballistic missile test that violates UN resolutions imposed after the North's nuclear tests in 2006 and 2009.

"We see it as a weapon of an enemy state, and since this launch was in violation of UN resolutions, we do not have to return it even if North Korea demands," said Kim.

The UN Security Council has condemned the launch and warned of possible measures over what the US called a "highly provocative" act.