Unification Minister Chung Dong-young says the construction of light-water reactors is the most difficult task in resolving North Korea’s nuclear standoff.

In a speech Thursday, Chung said some 15 disputed points remain in the six-way nuclear talks, with the North's right to peaceful use of nuclear energy and the light water reactor project being the key issues.

The North has insisted on the right to peaceful nuclear development and building light-water reactors for energy.

Chung, however, said the talks will mark a watershed in Korean history, adding he is optimistic about reaching an agreement on peaceful nuclear activities.

He said the parties to the talks have agreed on many issues such as recognition of the North's sovereignty, a security guarantee and Pyongyang's official ties with Washington.