Prospects of finalising a new binding agreement on climate change by the end of the year are "slim", according to UN climate convention chief Yvo de Boer.
He was speaking at the first UN climate talks since the Copenhagen summit. A negotiating process was agreed, but big divisions remain between nations.
The EU vowed to step up efforts to achieve a legally binding treaty.
Analyses show pledges in Copenhagen are not likely to keep the global average temperature rise below 2C (3.6F).
"We see that with current pledges, we wouldn't reach that objective - this is why we were here, and this is why we are working to increase ambition," said Spain's delegation chief Alicia Montalvo Santamaria, speaking for the EU.
Grenada's Ambassador Dessima Williams, who chairs the Association of Small Island States (Aosis), said that despite agreement on how negotiations should proceed through the year, there were still hurdles to cross in terms of what a new global deal might look like.
"The question now is whether or not there will be a sense of ethical commitment [from high-emitting countries]," she told BBC News.
"The situation of climate change is not as dramatic as an eathquake, but it is of equal proportion, [and] there's a greater burden of responsibility on the major emitters."
Dangers of symmetry
On the final evening of the three-day meeting, delegates took more than four hours to agree apparently simple matters such as how many times to meet over the year, and how the chair should write a draft negotiating text.
The protracted wrangles were rooted to a large extent in the debris of Copenhagen, in particular in the mistrust engendered when on the final day, a small group of countries wrote and then agreed the Copenhagen Accord, a political declaration entailing voluntary carbon curbs from major emitting countries.
Here, developed nations such as the US and Australia lobbied for the accord to be incorporated into any new global agreement.
But developing countries decried it as far too weak, and objected to the "undemocratic" nature of the process.
"It has heightened the feeling of distrust within the process," Mr de Boer told BBC News.
About 110 countries have endorsed the accord. But many of them have added the caveat that they see it only as a step towards a global, binding treaty, and that they want the treaty agreed by this year's summit, to be held in the Mexican resort of Cancun in November and December.
The chances of that happening, said Mr de Boer, were "very slim".
"I think that developing countries will want to see what the nature of an agreement is going to be before they will be willing to turn it into a legally- binding treaty, so that basically means a two-step process," he said.
Nevertheless, US delegation chief Jonathan Pershing found much to like in the meeting here.
"If I think about the contrast between this meeting and Copenhagen... this was pretty good," he said.
"I think there's been time for the dust to settle around Copenhagen, for people to look back and think 'it's not perfect, but it advances the ball substantially - let's work with it'.
"I don't think people want this to fail."
Mr de Boer flagged up the US demand for "symmetry" as a potentially major stumbling block.
In order to placate domestic concerns about losing competitiveness, the US is for example demanding that China and other major developing countries should be subject to the same regime on verifying emissions curbs as industrialised nations.
"What the US has also indicated is that it would want to be treated on a par with major developing countries, and that I think is going to be very difficult," said Mr de Boer.
Negotiators eventually decided here that there should be three more negotiating meetings between now and the Cancun summit.
"It is a very involved process - it is not a sprint, it is a decathlon," noted India's delegate Vijai Sharma.
Developing countries are looking for clarity within months on "fast-start finance" - the pledge in the Copenhagen Accord under which developed nations, principally the EU, Japan and the US, will release $30bn over three years to developing nations.
The US made it clear that any country not endorsing the Copenhagen Accord would be unlikely to receive US funding.
"In our view, fast-start financing was explicitly an agreement of the Copenhagen Accord, and those countries that are part of the accord should expect to see that finance," said Mr Pershing.
Last week, the US said it had withheld money from Bolivia and Ecuador because they had not endorsed the accord - a development condemned as "arm-twisting" by some activists.
The EU's approach is not completely clear. The European Commission's lead negotiator Artur Runge-Metzger told reporters it "would be hard to think that money would flow to countries not associated with the accord"; but later Ms Montalvo Santamaria said endorsing the accord was "not a conditionality" to receiving the cash.
Before June, the EU aims to produce a report outlining how its share of the first year's fast-start money - 2.4bn euros - was being allocated and spent.
The next month sees an intense flurry of non-UN climate meetings.
Next week, the US hosts a meeting of the Major Economies Forum, which brings together 17 of the biggest-emitting countries.
That is followed closely by Bolivia's World People's Conference on Climate Change and the Rights of Mother Earth; then, in early May, Germany hosts a ministerial meeting of a group of selected countries.
Kathrin Gutmann from WWF said leaders had to understand that current pledges were not enough to meet their own targets.
"The important thing is not to legitimise the proposals we have as being enough," she said.
"There's no political will to increase pledges, there's no debate happening, and that's not acceptable."