North Korea has been developing a new long-range ballistic missile in a separate programme from the one that led to a failed rocket launch this week, a South Korean TV station reported on Saturday.
YTN quoted an intelligence source as saying the communist state carried out four tests over 16 weeks until early this year to develop an inter-continental missile at a test facility at Musudan-ri on the northeastern coast.
The tests were aimed at improving engines and propellant fuel for the missile, code-named KN-08, the source said.
The experiments took place amid ongoing talks with the United States that resulted in a February deal under which the North agreed to freeze its nuclear and long-range missile tests in return for food aid, YTN said.
North Korea, which admitted its long-range rocket launch failed on Friday, has been developing missiles for decades both for what it terms self-defence and as a lucrative export commodity.
It said Friday's launch was aimed at putting a peaceful satellite in orbit.
But the United States and its allies condemned what they see as an apparent disguised test of ballistic missile technology in defiance of UN resolutions. Washington has halted plans to send food aid to Pyongyang.