Parties to the six-party nuclear talks are reportedly considering holding "unofficial" or "preliminary" peace treaty talks when the nuclear talks resume.

A diplomatic source in Seoul said Monday that the peace treaty issue is clearly stipulated in the September 2005 joint declaration.

He added that active discussions are under way among related parties on how to sequence peace treaty talks, the provision of economic and energy aid, and normalizing diplomatic ties with the United States in return for the North's denuclearization.

The source also said that conditions for opening peace treaty talks are the main concern, adding there's a general understanding that two Koreas, the U.S. and China should participate in the talks. The critical issues are reportedly when and how to initiate the talks.