Senior U.N. officials were forced today to defend the aim of this month's sustainable development conference in Brazil during their first press briefing there.

The officials addressed the media on what was the first day of negotiations in Rio de Janeiro over an official outcome document to frame the Rio+20 summit, which formally swings into gear June 20. Delegates are meeting this week in pre-conference talks in an attempt to agree on an agenda.

The document has been in the works for months, but with U.N. member nations and interest groups having failed to ratify the agenda in New York, parties had to schedule rushed meetings this week in an attempt to set the stage for "deliverables" from the conference.

The outcome document started out several months ago as a 19-page document only to seesaw between 100 and 200 pages more recently. Ideas have ranged from bold proposals such as establishing a World Environment Organization and ending fossil-fuel subsidies to more subdued ideas like drafting a set of sustainable development goals, or SDGs, that would be a broad call to action but leave the nitty-gritty of implementation to later meetings.

Under questioning from local reporters or media who arrived early in Rio, Sha Zukang, secretary general of the summit, insisted a "transformative outcome" is still possible despite the change in tone at U.N. conferences from treaty building to a more open-ended approach.

As he has before, Sha explained that the outcome would not take the form of a legally binding treaty. Instead, a "compendium" of voluntary commitments from countries, businesses and other stakeholders is likely to emerge by conference's end, as well as a pact on sustainable development goals, he said.

"I should be honest with you, negotiations in New York were challenging," he said. "We must drastically accelerate the pace."

Sha's comments come as about 50,000 visitors are expected in Rio, including more than 130 heads of state and U.S. Secretary of State Hillary Rodham Clinton.

Just what those heads of state will be negotiating by this time next week is unclear. The officials seemed well aware of that reality but nevertheless pressed ahead with the belief that Rio+20 can be a different kind of conference than its predecessor in the same city 20 years ago, which at the time set the stage for a major climate change treaty in Kyoto and the United Nations' biodiversity convention.

Brazilian Ambassador Luiz Figueiredo Machado, the country's lead negotiator at the talks, told reporters that the 1992 Rio Earth Summit benefited from policy proposals that "were ripe for adoption" as the conference opened. This led to opening both the climate change and biodiversity conventions to signature and produced binding treaties on both.

This time, Machado said, "We are not at the end of processes. We are at the start of new processes of new implementation."

"There is a fundamental difference between the two situations," he said.

Machado went on to argue that there is little room for new treaties brokered by the United Nations and asked diplomats to focus on how they could implement past agreements, many of which have failed to live up to their promise.

"What we need at this point in time is to implement what we had agreed to back in 1992," he said. "We have to look back and see what was done and not done."

Sha added that draft SDGs are in the works that are modeled on the United Nations' Millenium Development Goals, which expire in 2015. These SDGs would urge nations to take broad topics like poverty eradication, food security, sustainable cities, oceans protection and smart energy development more seriously, he said.

Those already on the ground in Rio were a bit more optimistic. Joanna Benn, a senior officer in international policy at Pew Environment Group, described the pre-conference talks as "intensifying" and said a number of agreements are nearly completed on oceans and fisheries issues, to name a few.

For instance, Benn said the outcome document draft now includes a proposal to eliminate "illegal, unregulated and unreported fishing" by 2020. Also currently in the text is a directive to regional fisheries management organizations to get tough on U.N. recommendations and undertake independent reviews.

Stakeholders have been given until Friday to finish the outcome document. After that, the Brazilian government will host "dialogue days" on 10 key areas from Saturday to Tuesday, then the formal conference begins and lasts until June 22.

Sullivan reports from New York.