US officials say North Korea may have used Iranian rocket technology for the third stage of the missile that fell into the sea shortly after launch on Friday.
The officials spoke to NHK about their analysis of the ballistic missile that the North insisted was a satellite-carrying rocket.
The officials said North Korea may have built the missile by improving an advanced version of the Taepodong 2 rocket that the country launched in 2009 and is believed to have fallen into the Pacific Ocean.
The Taepodong 2's 3rd stage, which was a Chinese Long March 1 rocket, is believed to have failed to separate.
The officials said North Korea this time very likely introduced technology from Iran's Safir rocket for the 3rd stage, where a satellite is believed to have been mounted.
The 2 rockets had similar shapes and sizes, and had first stages consisting of 4 middle-range Rodong missiles.
The officials say that behind the North's decision to use Iranian technology were cooperation between the 2 countries in developing a ballistic missile and Iran's successful satellite launch in February.
US intelligence authorities have expressed concern that North Korea and Iran may deepen their cooperation in the nuclear sector. Both nations face international criticism over their nuclear development.