US delegates have begun a fourth round of bilateral talks with North Korea, in an effort to break the deadlock over its development of nuclear weapons.
Christopher Hill said he still had 'plenty of patience'
The talks are on the sidelines of the main six-nation discussions in Beijing.
US envoy Christopher Hill and North Korean Vice Foreign Minister Kim Kye-gwan are trying to agree on a plan to end the North's nuclear programme.
Both nations have put fresh demands on the table, although they are still a long way off reaching a deal.
The US delegation's decision to engage the North Koreans directly is a significant departure from its previous approach, according to BBC correspondent Charles Scanlon.
The other countries attending the talks - South Korea, Japan, China and Russia - have been pushed to the sidelines by the increasing focus on the bilateral negotiations, he says.
In previous rounds the US was wary of direct contact, preferring to use the six-party format to put regional pressure on North Korea.
Oct 2002: US says North Korea is enriching uranium in violation of agreements
Dec 2002: North Korea removes UN seals from Yongbyon nuclear reactor, expels inspectors
Feb 2003: IAEA refers North Korea to UN Security Council
Aug 2003: First round of six-nation talks begins in Beijing
Feb 2005: Pyongyang says it has built nuclear weapons for self-defence
China's foreign ministry spokesman, Qin Gang, said he was encouraged by the bilateral discussions.
"They can sit down and have a prolonged and businesslike discussion. It's far too early to look at the results, but if you look at the forms, I think that this is a good sign," he told the BBC.
But despite the efforts at compromise, fundamental differences remain between the US and North Korea.
Pyongyang wants diplomatic relations with the US and a peace agreement, in addition to security guarantees and economic help.
For its part, Washington has called for concessions on North Korea's development of ballistic missiles and its human rights record.
These demands are in addition to Washington's most significant requirement - that Pyongyang agree to the verifiable dismantlement of its nuclear weapons programmes.
North Korea objects to US demands that it make the first move by scrapping its nuclear weapons facilities. Instead it wants a step-by-step process in which it receives progressive rewards and incentives.
The North has also continued to reject American allegations that it is running a secret enriched uranium programme in addition to its well-known plutonium plant at Yongbyon.
That dispute provoked the current crisis, which began nearly three years ago and has blocked diplomatic progress since.
'Plenty of patience'
Despite the obvious gulf between the two sides, there has been a more positive atmosphere at this fourth round of six-party talks, our correspondent says.
Some participants have even suggested the talks could continue into next week.
"We'll just keep at it just as long as it's useful to keep at it. I've got plenty of patience," Mr Hill said on Friday.
The previous three rounds broke up after just three days without any written agreement.
But the stakes are high. Failure to make progress this time could fatally undermine diplomatic efforts and open the way for more coercive US action, analysts say.