US officials say the North is deliberately blowing hot and cold
The White House has described as "positive" the six-nation talks in Beijing on North Korea's nuclear ambitions.
The first indications of progress emerged on the second day of the summit, when the participants agreed to reconvene for more talks.
However, North Korea has been accused of giving out mixed messages - on the one hand threatening to conduct nuclear tests, while on the other speaking of disarmament.
This week's talks are the first formal discussions on the crisis since April, and the first to include South Korea, Japan, Russia and host nation China.
"The assessment from our team that's on the ground... is that this is a positive session that they've been having," said White House spokeswoman Claire Buchan.
"We believe there's been excellent co-operation... between the United States and China, Japan, South Korea and Russia," she added.
However, she refused to be drawn on reports by US officials that Pyongyang's envoy told delegates North Korea may formally declare itself a nuclear power and carry out tests.
"North Korea has a long history of making inflammatory statements that serve to isolate it from the rest of the world," she said.
An unnamed US official told Reuters news agency that North Korea was playing "a calculated game of confusing the adversary" with both belligerent and conciliatory statements.
"We heard both things," he said. "This is characteristic of the North Koreans. They are all over the place."
Although there is no sign of a diplomatic breakthrough over the 10-month old crisis, Russian and Chinese officials said the participants had agreed to reconvene for more talks.
Earlier on Thursday, a statement carried by the Chinese official news agency indicated that some common ground had been found.
"The parties reiterated that denuclearisation of the Korean peninsula is the common goal of all sides, and the nuclear issue should be resolved peacefully through diplomatic means," Xinhua said.
But the real sticking point is how this is achieved.
North Korea says it will not give up its nuclear programme without a guarantee that the US will not attack, but Washington is unwilling to agree to a non-aggression treaty while North Korea clings to its nuclear ambitions.
At Thursday's session, the delegates gathered around their specially commissioned hexagonal table for nearly four hours of formal talks, in what Xinhua described as a "frank atmosphere".
They spent much of the time responding to each other's opening statements, according to South Korean foreign ministry official Jeong Woo-jin.
The Russian negotiator, Alexander Losyukov, said it was likely that a communiqué - bringing together the discussions of the last two days - would be adopted on Friday, the last day of talks.
He said it was agreed that further talks should take place in the next two months, although a South Korean Foreign Ministry official, Wie Sung-rak, would only say that there was a consensus the six-party discussions should continue.