The United States' lead envoy expressed confidence Tuesday about the prospects of restarting long-stalled nuclear negotiations after two days of "very positive" talks with North Korea.

The parties were able to narrow some differences, although more time was needed to reach an agreement, Stephen Bosworth, outgoing US special representative told reporters following the talks.

"We had some very positive and generally constructive talks with the DPRK delegation. We narrowed differences on several points and explored differences on other points," he said.

Bosworth noted that there remained sticking points, "not all of which can be overcome quickly."

However, "I am confident that with continued efforts on both sides we can reach a reasonable basis of departure for formal negotiations for the return of the six party process," he added.

Together with his successor Glyn Davies, Bosworth met the North Korean delegation led by first vice minister Kim Kye-Gwan over two days in Geneva. The meeting, which took place first at the US embassy on Monday, rotated to the North Korean mission on Tuesday.

It coincided with a visit by China's vice premier Li Keqiang to Pyongyang, where he met North Korean leader Kim Jong-Il, according to Chinese state media.

Kim told Li that Pyongyang "hopes the six-party talks about the nuclear issue on the Korean peninsula should be restarted as soon as possible", China's official Xinhua news agency reported early Tuesday.

The "principle of simultaneous action" should apply, it quoted Kim as saying -- a reiteration of the North's stance that the negotiations should begin again without preconditions.

The North formally quit the six-party forum in April 2009, a month before staging its second atomic weapons test. It has since repeatedly said it wants to come back, but Washington is demanding a physical sign of sincerity first.

Li, widely expected to be China's next premier, said Beijing was "working for positive achievements, creating conditions for reopening the six-party talks at an early date", Xinhua said.

In Washington, a senior State Department official sought to downplay any hopes of an imminent breakthrough.

The North Korean team that came to Geneva "obviously has to go home now and has to consult with their leadership. We all know how the North Korean system works," said the official who requested anonymity to discuss the talks more frankly.

He added that it would probably be "a matter of weeks and months before we're going to be able to really know where we're going next on this."

North Korea's chief delegate however maintained a positive outlook, noting "big progress" and that the two sides had agreed to meet again for discussions on remaining issues.

"We focused on discussing trust-building measures to improve North-Korea-US relations as agreed in the first round of talks (in New York in July), said Vice Foreign Minister Kim Kye-gwan, the Yonhap news agency reported.

China, which is Pyongyang's closest ally and a major economic partner, has hosted the six-party forum since 2003, which includes the two Koreas, Russia, the United States and Japan.

In September 2005, the North agreed to abandon all nuclear weapons and programmes in return for security guarantees, energy aid and a peace pact formally ending the 1950-53 war and diplomatic ties with the United States.

In 2007 it shut its plutonium-producing Yongbyon reactor. But during the following year the process began to melt down amid mutual accusations of bad faith.

In April 2009 Pyongyang walked out of the six-party forum, a month before staging its second atomic weapons test.

The North's deadly artillery attack last November on a South Korean island further complicated efforts to restart nuclear dialogue.

But a surprise meeting between nuclear envoys of South and North Korea on the sidelines of an Asian security conference in Bali, Indonesia led to the first round of direct US-North Korea talks in New York in July.

The second set of discussions were held after the two parties agreed to resume searches for the remains of Americans killed in the 1950-53 Korean War after a six-year hiatus, in a further sign of easing tensions between the two sides.