COPENHAGEN -- Top U.S. climate delegate Todd Stern said late Monday that while some progress has been made so far in talks here, "there's still a long way to go if we're going to reach the kind of agreement" that major players would back.

The core issues remain unresolved for U.S. negotiators, including convincing major developing nations to "indicate their resolve" to implement emission reduction goals with "adequate transparency," Stern said. He said the progress made over the weekend was "clarifying and defining more clearly where there are differences."

Former Danish climate minister Connie Hedegaard, who is president of the conference here, started informal talks with 48 ministers over the weekend. Stern said Saturday night and particularly Sunday "were constructive."

The baby steps come as more than 100 heads of state are slated to start arriving here Wednesday. President Obama is planning to arrive "reasonably early" Friday morning, the last day of the talks, Stern said. He said there is still broad desire to get issues resolved before Obama gets here and to have anything "kicked upstairs." But, he said, his view is "there will be an issue or two that does require the engagement of leaders."

"The pending arrival of well over a hundred leaders absolutely affects the negotiation" by putting pressure on the negotiators, Stern said. "This is a completely unprecedented situation."

He said today's African-led mini-protest on the process to follow for the rest of the way did not help. "We don't have very much time," Stern said. "The clock is definitely ticking. There was some time lost there."

He added: "Process can become your enemy if there's an insistence on discussing and going around on that process too much. There's a real cost."