The chief US negotiator on North Korea said Thursday that Washington remained open to new six-party talks with the Stalinist state, but that a long-range missile test could derail future negotiations.

Assistant Secretary of State Christopher Hill told US lawmakers that he remained uncertain whether North Korea intended to resume the stalled six-party talks that also include China, Japan, South Korea and Russia.

"The question that keeps me up at night is: are they serious?"

He said that a potential North Korea missile test would have "the opposite effect" and actually compromise rather than strengthen the regime's future security.

Hill stressed: "While a launch would raise questions about the future of the six-party talks, the US remains prepared to return to the table, with no preconditions."

Pyongyang is believed to be preparing to launch a long-range Taepodong-2 missile, capable of reaching Japan, Alaska or possibly Hawaii.