The U.S. State Department says North Korea has not stopped efforts to restart its nuclear program despite the latest diplomatic effort to revive a deal on dismantling the country's nuclear program.

State Department spokesman Robert Wood said Friday that U.S. envoy Christopher Hill was not able to convince Pyongyang to halt steps to reverse disablement of its nuclear facility at Yongbyon. Wood said the latest confirmation of the North's ongoing nuclear activity was received late Thursday during Hill's visit to Pyongyang.

After returning to Seoul on Friday, Hill said he had conducted "lengthy" and "substantive" talks with North Korean officials. But he declined to say if any real progress was made during his three-day visit.

The U.S. assistant secretary of state said he held talks in Pyongyang with his counterpart, Kim Kye Kwan, as well as North Korea's foreign minister and a Korean People's Army general.

Hill said he will not provide further details on his meetings with North Korean officials until after he has spoken to the U.S. secretary of state and the other countries involved in talks to end Pyongyang's nuclear program.

The U.S. envoy briefed South Korean and Japanese counterparts, Kim Sook and Akitaka Saiki, after his return to Seoul. He is scheduled to return to Washington late Saturday after meeting with Chinese and Russian officials in Beijing.

The United States wants an international verification of North Korea's compliance with a six-nation disarmament deal. But North Korea says verification is not part of the current stage of the agreement.

Last year, North Korea agreed to end its nuclear program in return for energy aid and diplomatic benefits from the United States, South Korea, Japan, Russia and China. However, in recent weeks, Pyongyang began restoring equipment at its main nuclear facility after Washington declined to remove it from its list of countries sponsoring terrorism.