South Korea stepped up an operation Saturday to retrieve debris from North Korea's failed rocket launch, officials said, apparently in competition with Russia and China.

The operation began on Friday after the North's rocket exploded mid-air about two minutes after blast-off and its debris came down in the Yellow Sea.

South Korean officials have said some large chunks may have fallen into the country's exclusive economic zone off the west coast.

"There has been no luck yet in our operation," a defence ministry spokesman told AFP, adding South Korean ships were combing a wide area well off two southwestern ports -- Pyeongtaek and Gunsan.

Warships from China, Russia and the United States have also launched their own operations, he said, declining to give details.

Cable news network YTN said the South's operation involved a dozen navy ships, backed by helicopters.

The operation came despite North Korea's warning last week that any attempt to retrieve its rocket debris would face a "ruthless" retaliation.

Pyongyang has said the rocket would place a satellite in orbit for peaceful research purposes, but Western critics and Seoul said the launch was a thinly veiled ballistic missile test, banned by United Nations resolutions.