The Obama administration of the United States on Friday voiced its deep concern over Pyongyang's declared progress on its nuclear weapons program, vowing to continue tough international sanctions.

In a letter to the president of the UN Security Council (UNSC) on Thursday, the Democratic People's Republic of Korea (DPRK) claimed that the reprocessing of spent fuel rods "is at its final phase and extracted plutonium is being weaponized."

"We are very concerned by these claims that they're moving closer to the weaponization of nuclear materials," State Department spokesman Ian Kelly told reporters, but refused to comment on the veracity of Pyongyang's claims.

White House spokesman Robert Gibbs said Friday that the United States would continue to "be committed to ensuring that North Korea upholds its international obligations" and "strongly implement the sanctions that were approved."

In the letter, Pyongyang said it has been "prepared for both dialogue and sanctions," adding "if some permanent members of the UNSC wish to put sanctions first before dialogue, we would respond with bolstering our nuclear deterrence first before we meet them in a dialogue."

The UNSC adopted a presidential statement condemning the April 5 rocket launch by the DPRK and demanding the country "not conduct any further launch."

Pyongyang subsequently announced it was quitting the six-party talks on nuclear disarmament and would restart nuclear facilities in protest of the UN presidential statement. On May 25, the DPRK conducted its second nuclear test.

Responding to Pyongyang's behavior, the Obama administration has decided to extend economic sanctions by prolonging the national emergency on the DPRK and has vowed to enforce sanctions against Pyongyang set in the UN Security Council Resolution 1874.

(Xinhua News Agency September 5, 2009)