The UN's top climate change official on Monday called on governments to reach an agreement and deliver "significant and immediate action" as the UN Climate Change Conference opened in Copenhagen.

"The clock has ticked down to zero. After two years of negotiations, the time has come to deliver," said Yvo de Boer, executive secretary of the United Nations Framework Convention on Climate Change (UNFCCC).

De Boer spelled out three layers of action that governments must agree to: fast and effective implementation of immediate action on climate change; ambitious commitments to cut and limit emissions, including start-up funding and a long-term funding commitment; and a long-term shared vision on a low-emissions future for all.

"Copenhagen will only be a success if it delivers significant and immediate action that begins the day the conference ends," said de Boer.

He urged negotiators to focus on crafting "solid and practical proposals" that will unleash prompt action on mitigation, adaptation, finance, technology, REDD (Reducing Emissions from Deforestation and Forest Degradation in Developing Countries) and capacity-building. "Developing countries desperately need tangible, immediate action on these crucial issues."

"Solid, action-oriented proposals will give real meaning to the political commitment for success in Copenhagen that has been building momentum around the globe. Such proposals will also provide a strong foundation for further efforts," he said.