"Whether they'd prefer to do with all of us in the room,

and an opportunity to see us bilaterally, we're open to whatever can take this forward. What's most important is that they come prepared to really engage on the substance," the US official said. Nuland said the US and its international partners have made an offer to engage. "We have had three rounds of talks already. This will be the fourth one (in Almaty). We've been disappointed that those have not yet resulted in real, concrete progress," she said. "We've also said that in the absence of progress, in the absence of movement, we would continue to ratchet up the international pressure. So as long as Iran fails to address the concerns of the international community, we're going to have to continue to increase the pressure and isolate Iran internationally," the US official said. "So this is really a choice that Iran's leaders can make. They can have a better path for their people. But they've got to make the choice to come clean and answer the international community's concerns about their nuclear program, and that has yet to happen. We'll have another opportunity for them to do that in Almaty," Nuland added.