The United States has demanded that North Korea fully drop its nuclear weapons program by the end of 2008 under a joint statement signed in September last year.

Sources close to the six-way nuclear talks said Sunday that Washington told Pyongyang last week in Beijing that more sanctions will come if the North does not implement last year's agreement by that time.

The United States also reconfirmed its offer of a security guarantee, economic assistance and formal ties in return for nuclear dismantlement.

Washington's proposal is apparently reflective of its intent to give Pyongyang a set time to implement the agreement and to facilitate smoother discussions after the talks resume after a hiatus of more than a year.

The United States, South Korea and Japan drafted the proposal, which also reflects Chinese opinion.

Chief U.S. nuclear negotiator Christopher Hill conveyed the proposal to North Korean counterpart Kim Kye-kwan during the Beijing talks.