President Barack Obama announced Friday a "meaningful and unprecedented breakthrough" on a global effort to curb climate change. But Obama said, "It is going to be very hard, and it's going to take some time" to get to a legally binding treaty.

A deal reached by the United States, China, India, South Africa and Brazil includes a method for verifying reductions of heat-trapping gases, a senior administration official said.

The official, speaking on the condition of anonymity before Obama spoke, characterized the deal as a first step, not yet enough to combat the threat of a warming planet.

Under the agreement, the official said each country also will list the actions it will take to cut global warming pollution by specific amounts. The deal reiterates a goal that eight leading industrialized nations set earlier this year on long-term emission cuts and provides a mechanism to help poor countries prepare for climate change.