North Korea says it has successfully conducted experimental uranium enrichment and is in the process of building more nuclear weapons.

The North’s state-run Korean Central News Agency said Friday that the North’s mission to the U.N made the announcement official in a letter sent to the U.N. Security Council on Thursday.

In the letter, the North Korean mission said its reprocessing of spent fuel rods is at its final phase and extracted plutonium is being used to make weapons.

However, the letter added that the North does not object to denuclearization of the Korean Peninsula and the world as a whole, only that Pyongyang is opposed to the six-way nuclear talks which, the North said, were used to trample on Pyongyang’s sovereign rights. The letter said the denuclearization of the Korean Peninsula is closely related to U.S. nuclear policy on the North.

The North’s mission said Pyongyang has made clear the measures it would pursue to counter the new sanctions imposed by the U.N. Security Council. The mission said that if some Security Council members opt to seek dialogue while continuously seeking sanctions, the North would engage in dialogue while strengthening its nuclear deterrent.

The North’s mission then said that Pyongyang would not have conducted a second nuclear test in May if the council had not taken issue with the North’s April launch of a rocket for peaceful purposes.