The U.S. Department of State says it is very concerned by North Korea’s claim that it has reached the final stage of uranium enrichment.

But department spokesman Ian Kelly said the validity of Pyongyang’s claims remains unclear.

Meanwhile, White House spokesman Robert Gibbs said the United States will continue to implement U.N. sanctions regardless of the North’s renewed nuclear claim.

U.S. broadcaster Fox News reported that the United Nations may hold a meeting to discuss the issue as early as Saturday.