"We have - over the last week or so - seen progress in a number of areas, but we haven't seen enough of it.
"There is still an enormous amount of work and ground to be covered if this conference is to deliver what people expect it to deliver."
UK Prime Minister Gordon Brown will be one of the first leaders here, arriving on Tuesday evening.
Non-governmental organisations are protesting that many campaigners will be turned away from the venue.
Far more people have applied to attend the summit than Copenhagen's Bella Center can hold, and NGO numbers will be progressively reduced during the rest of the week, partly for security reasons as heads of state and government arrive.
On Monday, the talks were temporarily suspended after a delegation representing developing nations withdrew their co-operation.
Following the action by the African group, supported by the wider G77-China bloc of developing nations, some sessions ran long into the night as negotiators tried to make up lost time.
THE WEEK AHEAD
Tues 15 Dec - Prince Charles delivers a speech on the sustainability of human society; UK Prime Minister Gordon Brown arrives
Weds 16 Dec - High-level segment begins; protests expected inside and outside centre
Fri 18 Dec - More than 100 world leaders, including Barack Obama, attend closing session
The Danish conference hosts had been accused of trying to sideline negotiations on the Kyoto Protocol by packaging discussion of outstanding difficult issues from across the various strands into a single informal session.
Developing countries are adamant that developed nations still inside the protocol - all except the US - must commit to further emission cuts under its aegis.
After discussions with the Danes and UN climate convention officials, the informal talks were split as the G77-China bloc had demanded.
One group, chaired by Germany and Indonesia, is examining further emission cuts by developed nations under the Kyoto Protocol.
Another, chaired by the UK and Ghana, is looking at long-term financing to help poorer countries develop along "green" lines and protect themselves against impacts of climate change.
A senior Chinese source, meanwhile, confirmed to BBC News that China would not accept any money from the west for these purposes.
This is likely to carry political significance in the US, where some legislators are adamant that domestic carbon-cutting measures must not hand funds to the country set to emerge as its biggest economic rival.
Here, the positions of the world's two largest emitters are very much at odds, with China rejecting US demands that its emission curbs must be subject to international verification.
The US also rejected the notion that it would deepen its offer of cutting greenhouse-gas emissions.
"I am not anticipating any change in the mitigation commitment," US chief delegate Todd Stern told reporters, saying that Washington's stance had already been spelt out by President Barack Obama.
last month, the US administration announced a series of emission targets. It pledged to cut greenhouse gas emissions by 17% from 2005 levels by 2020, 30% by 2025, 42% by 2030 and 83% by 2050.
The final high-level segment of the summit, due to be attended by about 120 heads of state and government, will open on Tuesday evening with speeches from dignitaries including UN Secretary-General Ban Ki-moon, Danish Prime Minister Lars Lokke Rasmussen and Prince Charles.
Speaking to reporters before flying out from New York, Mr Ban warned that "time is running out".
"If everything is left to leaders to resolve at the last minute, we risk having a weak deal or no deal at all," he said.
Ban Ki-moon: "We must not exceed two degrees"
"And this would be a failure of potentially catastrophic consequence."
Prince Charles's speech will look at climate change in the context of wider concerns about the sustainability of human society.
He is expected to argue that although the human race has created the modern problem of climate change, the human race also has the capacity to solve it.
China has accused developed countries of backtracking on what it says are their obligations to fight climate change and has warned that the UN climate talks in Copenhagen have entered a critical stage.
Foreign Ministry spokeswoman Jiang Yu said there had been "some regression" on the part of developed countries, who had "put forward a plethora" of demands on developing countries.
Beijing's view is that the US and other richer nations have a historical responsibility to cut emissions, and any climate deal should take into account a country's development level.
Do you have a question for world leaders at Copenhagen? What would you ask them if you were at the summit? Send it to us. The best questions we receive will be answered by a panel of world leaders in a special televised debate.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.