Once again, U.N. climate change talks ended without a clear result, further hampering chances for a successful outcome of a major climate summit in Cancun, Mexico, in December.

The 2-week-long negotiations in Bonn, Germany, failed to bring the more than 180 nations together. Not that there hadn't been hope for a breakthrough. The 4,500 delegates came up with a draft treaty that was lauded by environmental groups but some last-minute changes to it at the request of Russia angered developing nations, which then refused to agree to it.

The United States and China also voiced their disapproval with the text, intended to be used as the basis for negotiations at the 16th Conference of the Parties to the U.N. Framework Convention on Climate Change -- or Cop 16 -- in the Mexican resort of Cancun in December.

Meanwhile, poor nations complained that they haven't yet seen anything of the $30 billion pledged in "fast-start" aid to help developing countries cope with climate change, to be delivered from now until 2012.

They also called for an update on where scientists stand regarding the latest climate change data to help guide the negotiations. The fifth assessment report of the Intergovernmental Panel on Climate Change won't be available until 2014 and individual elements of the fourth IPCC report, unveiled in 2007, have been criticized as too pessimistic. However, this request was denied by a few oil-producing nations, including Saudi Arabia and Venezuela.

Outgoing U.N. climate chief Yvo de Boer was nevertheless optimistic, vowing that the two additional talks scheduled before Cancun -- one more in Bonn in early August and another one in Beijing in October -- will produce an ambitious draft treaty holding a potential for consensus.

Climate negotiations have been deadlocked since Cop 15 in Copenhagen ended in acrimony.

Leaders jetting to Denmark couldn't agree on concrete emissions reduction targets or a way to measure them. The summit culminated in the publication of the so-called Copenhagen Accord, a weak declaration agreed between the United States, China, Brazil and South Africa after larger negotiations had broken down.

The accord wasn't adopted but merely noted by countries, many of which denounced it. While some of the elements of the accord have now made it into the draft treaty, industrialized and developing nations are still at odds over how to limit the global temperature rise to no more than 3.6 degrees Fahrenheit. A rise beyond that limit would result in potentially catastrophic consequences for humanity, with meteorological disasters increasing, scientists say.

Developing nations have resisted a legally binding treaty because they claim rich nations that have benefited from emitting during the past decades should shoulder more of the burden.

Industrialized countries argue the developing nations need to commit to concrete reduction targets to enable a global effort. The European Commission recently backtracked on a plan to unilaterally boost the bloc's greenhouse gas emissions reduction target from 20 percent to 30 percent.

The European Union has committed itself to reduce its carbon dioxide levels by 20 percent until 2020 and boosted that target to 30 percent if the world's other major emitters -- the United States and leading emerging economies such as India and China -- come together for a binding climate protection deal.

China recently overtook the United States as the world's biggest emitter of heat-trapping greenhouse gases but still emits far less on a per capita basis.