"We know that time is running out," Merkel told reporters after talks with Indonesian President Susilo Bambang Yudhoyono in Berlin.
"I will not hide the fact that I am somewhat nervous whether we will manage to achieve everything," she said when asked about her hopes for an agreement on limiting greenhouse gas emissions and parameters for slowing global warming.
Merkel urged developed and developing nations to make a "constructive contribution" to a deal, and renewed her calls for an international environmental agency and "binding" oversight of any pact in Copenhagen.
The summit, which reaches its climax Friday when 120 heads of state huddle in the Danish capital, aims to agree an outline deal of national pledges to curb carbon emissions and set up a mechanism to provide billions of dollars in help for poor countries in the firing line of climate change. But deep divisions remain over how to split the tab.
Indonesia hosted climate change talks on Bali island in 2007 and has been a key advocate for ambitious plans to help avert climate change by paying countries to conserve carbon-absorbing forests. But the country is also the third biggest emitter of greenhouse gases, after China and the United States - 80 percent of its emissions are the result of deforestation.
Earlier, Merkel met representatives from eight Pacific island nations, many of which are among the hardest hit by climate change.
The envoys "pressed for that reason for an ambitious result of the Copenhagen summit and for ambitious climate goals in future," the German government said in a statement.
Merkel told the group that Germany would press for an agreement that limited global warming to 2.0 degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial times in a bid to stave off an environmental catastrophe.