The United States, North and South Korea, Russia, Japan and host China are taking part in the three-day talks, intended to begin fleshing out a basic accord struck at the last round in September, but their success hinges on overcoming distrust between Washington and Pyongyang.
"The DPRK needs to understand we need to move swiftly on denuclearisation," chief US negotiator Christopher Hill told reporters, referring to Pyongyang by its official name, the Democratic People's Republic of Korea.
Hill described progress as "very good and substantive".
"We want to make demonstrable progress in the next few weeks," Hill said after bilateral meetings with the five other parties and sharing dinner with his North Korean counterpart.
But Hill also told North Korea that its nuclear plant at Yongbyon had continued to operate since the joint statement was signed on September 19. "In a sense, the problem has actually gotten worse," he said.
He also emphasised that the United States and North Korea remained at odds over many aspects of a potential agreement - including the timing and order of denuclearisation.
Earlier on Wednesday, Hill reiterated Washington's position that Pyongyang could not receive the reward of a light-water reactor for atomic energy until it had disarmed and opened up to nuclear inspectors.
In Washington, US President George W. Bush restated concerns about North Korea's human rights record, but urged patience at the talks and said the main goal was ending the North's nuclear weapons ambitions.
The last round was marked by increased one-on-one contact between the United States and North Korea that was seen as key to reaching the September agreement.
But Pyongyang said on Wednesday Washington could not be trusted, despite affirming at the last round it would not invade.
"The US words do not agree with its deeds at all," said a commentary in Rodong Sinmun, the North's communist party paper.
"The US is whetting its sword for invading the north behind the scene despite its oft-repeated talk that 'it has no intention to invade the north'," said the commentary, which was carried by the official KCNA news agency.