Ambassador Susan Rice said Afghan forces are taking on more security responsibility in the country as international forces look for the exit doors after more than 10 years at war.
"While transitioning security responsibility to the Afghans is necessary for stability, it is not itself sufficient," she said. "In this regard, an inclusive and credible presidential election in 2014 is critical for the country's future and to sustaining international assistance to the people of Afghanistan."
Afghan presidential elections in 2009 and parliamentary elections in 2010 were mired by allegations of fraud. Rice said the next round of election must be legitimate and accepted by the Afghan people.
The U.N. Security Council voted to extend the mandate for the U.N. mission in Afghanistan for another year. U.N. special envoy to Afghanistan Jan Kubis said the country wasn't yet out of the woods.
"There are a lot of questions and uncertainties practically concerning all areas of life and work in Afghanistan," he said.
Rice added the U.S. government backed plans to set up a liaison office in Qatar to work with Taliban members who renounce violence. Afghan President Hamid Karzai accused Washington of colluding with the Taliban, an allegation described as baseless.