VIENNA — On the eve of the opening round of negotiations to get a comprehensive nuclear deal with Iran, a US diplomat said the process would be "complicated, difficult and lengthy," but that it represented "the best chance" the world has had in a decade to resolve concerns about Iran's nuclear program.

"These next days will be the beginning of what will be a complicated, difficult and lengthy process," the senior US Administration official, speaking not for attribution, told journalists here Monday night. "When the stakes are this high, the devil truly is in the details. It can't be done in a day, a week, or a month."

"The aim is to move in a...deliberate manner to get the job done," the official said. "We need to build on the progress of [the] first step to get a comprehensive agreement that addresses all of our concerns."

The US official anticipated that the coming months of negotiations would likely have "some ups...and many downs" along the way, but said negotiators were going into them "clear-eyed, focused and determined...to see what can get done." She did not rule out that an agreement could be reached within six months. President Obama has put the odds of getting a final deal at fifty/ fifty, the US official noted.