WASHINGTON—The United States on Friday urged North Korea not to continue preparations to restart its nuclear reactor, saying the country must decide whether it wants to have a better relationship with the world or “keep themselves isolated.”

North Korea said earlier in the day it was making “thorough preparations” to restart the Yongbyon reactor and accused the United States of failing to fulfill its obligations, adding another hurdle to already stalled international nuclear disarmament negotiations.

State Department spokesman Sean McCormack told reporters that North Korea has been “getting closer and closer” to the point where it will restart the reactor but has not gotten there yet. “We would urge them not to get to that point,” he said.

The North did not say when the reactor might begin operating again. The comments marked the first time the North has confirmed a reversal of steps taken since last year to disable its nuclear program. North Korea is angry over Washington’s refusal to quickly remove it from a U.S. terrorism blacklist.

Asked why the North should deal with the Bush administration since it is in its final months in office, McCormack said, “I don’t know who the next president, who the next secretary of state is going to be, but I would wager that they’re not going to get a much different deal from the next administration as they’re getting from this administration.”

He also noted existing U.N. Security Council resolutions “that hold penalties for North Korea should they go down the pathway of, for example, missile testing, for example, further testing of any nuclear devices.”

Under a 2007 pact, North Korea pledged to disable its nuclear program as a step toward its eventual dismantlement in exchange for diplomatic concessions and energy aid.

In late June, North Korea submitted a long-delayed declaration of its nuclear activities and destroyed the cooling tower at Yongbyon in a show of its commitment to the nuclear deal. But the accord stalled in mid-August when Washington refused to take North Korea off its list of states that sponsor terrorism. The U.S. says the North first must accept a plan to verify its nuclear declaration.