SEOUL, South Korea  North Korea on Monday proposed talks with the United States to reach a formal peace treaty that would replace the truce that halted the Korean War 57 years ago, indicating that it would not give up its nuclear weapons until Washington signed such an accord.

North Korea said peace talks should be held either as part of the six-nation talks that focus on ending its nuclear weapons program or as a separate negotiation. But the North also warned that it would not return to six-nation talks  from which it withdrew last April  unless the United Nations lifted sanctions imposed after the North’s nuclear and ballistic missile tests last year.

The North had previously proposed peace negotiations with the United States and South Korea. But its latest overture came as it was trying to shift the focus of the six-nation talks, where a peace treaty had been set aside until North Korea made significant progress toward dismantling its nuclear weapons program.

“If a peace treaty is signed, it will help resolve hostile relations between North Korea and the United States and speed up the denuclearization of the Korean Peninsula,” the North Korean Foreign Ministry said in a statement carried by the North’s state-run news agency, K.C.N.A.