The United States says documents handed over by North Korea detail its weapons-grade plutonium program as far back as 1986.

The US says they are an "important first step" in getting a full declaration of the North's nuclear activities.

The documents were given to the State Department's Korea expert, Sung Kim, who carried them over the heavily fortified demilitarized zone into South Korea on Saturday.

Under a six-country deal, North Korea was required to provide a full accounting of its nuclear program.

In addition to the declared plutonium operation, Washington says the declaration must clear up suspicions about alleged secret uranium enrichment and about suspected proliferation to Syria.

The North denies both activities.

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