Ban Ki-moon says Rio+20 global development summit negotiations are bogged down on minor details and narrow national interests. Photograph: PETER DEJONG/AP

The United Nations chief, Ban Ki-moon, held out little hope on Thursday of an historic outcome at the Rio global development summit, now less than a month away, admitting negotiations had been "painfully slow".

The warning was the latest from United Nations officials and others involved in preparations that the summit, known as Rio+20, is unlikely to replicate the breakthrough achievements of the original environmental gathering in the city in 1992.

Ban, who has made sustainable development and climate change his signature issues as secretary-general, was candid about the difficulties of having world leaders engage with Rio.

"The negotiations have been painfully slow," he told a group of journalists at the United Nations foundation on Thursday.

The pace was so sluggish, in fact, that Ban prevailed on the international community to agree to an extra five days of talks, from 29 May to 2 June. The last-minute talks were aimed at getting at producing a face-saving outcome for a summit, which so far has failed to engage world leaders.