Progress was made at talks between the US and North Korea aimed at reviving the stalled nuclear deal but more headway is needed, the US envoy said.
Mr Hill says there needs to be faster progress on the nuclear deal
Christopher Hill described his talks with counterpart Kim Kye-gwan in the Swiss city of Geneva as "substantive".
But he urged the North Korean side to "pick up the pace" of the negotiations.
North Korea agreed last year to end its nuclear activities in return for economic aid, but the deal is currently deadlocked.
The communist nation has closed its Yongbyon nuclear reactor, but it missed a year-end deadline to provide a complete declaration of all of its nuclear activities.
In a late-night news conference, Mr Hill said the two sides had "had good discussions on all substantive issues".
At issue is whether or not Pyongyang has a secret programme to enrich uranium for weapons purposes, and whether it has transferred nuclear technology overseas.
North Korea denies both allegations and says it has already given Washington full details of all its nuclear activities.
"We have really had to challenge and to work with them on [the two issues]," Mr Hill said.
"I'm not really in a position to tell whether we've resolved that except to say that I think we've made some progress today."
He also called on North Korea to accelerate the pace of the talks.
"We are already in March and we have some ambitions for trying to get through this in 2008. There is no question we need to move faster," he said.
Mr Kim told reporters he was "satisfied" with the talks.
The two men would now return to their respective capitals to brief top leaders, Mr Hill said.
North Korea carried out a nuclear test in October 2006.