North Korea has been working on its missile programme
North Korea is trying to develop a nuclear warhead that would fit on to a missile, South Korea's top military official has said.
Gen Kim Tae-young told a parliamentary committee that the North was believed to have enough plutonium to make six or seven warheads.
He said it was not clear if Pyongyang had succeeded in developing a warhead.
North Korea recently back-tracked on an aid-for-disarmament deal, saying the US had not met its obligations.
South Korean media have also reported that the North test-fired one or two short-range missiles into the sea off the west coast of the Korean peninsula on Tuesday.
The North is also working on a long-range missile programme.
"I understand that North Korea is working to develop a small nuclear warhead which can be loaded into a missile," Gen Kim Tae-Young was quoted by South Korean media as saying.
"As I said earlier, it is certain that North Korea possess plutonium. It is certain the North has enough plutonium to make six to seven nuclear weapons, but it is not clear whether it has produced nuclear weapons," he said.
Gen Kim is the chairman of South Korea's Joint Chiefs of Staff - the country's highest-ranking military officer.
North Korea tested a nuclear device in October 2006, and some analysts estimated it had built eight or more atomic bombs.
But it was not believed that any of the bombs were small enough to be fitted onto a missile.
North Korea agreed last year to shut down its plutonium-producing reactor at Yongbyon in return for fuel aid.
But a disagreement over how to verify Pyongyang's account of its nuclear activities has led the US to delay removing North Korea from its list of state sponsors of terrorism.
In return, North Korea said it would take steps to restore the Yongbyon reactor to plutonium production.