GENEVA -- The U.S. and five other world powers go to the table with Iran today to demand a freeze of its nuclear activities, and a senior U.S. official said Washington may seek rare face-to-face talks with Iranian diplomats.

Even as they prepared for today's talks, the U.S. and its allies were contemplating new and tighter sanctions on Iran, a clear signal of expectations that the negotiations may again end in failure.

Secretary of State Hillary Rodham Clinton suggested all six -- the five permanent U.N. Security Council members and Germany -- were of one mind on the need for Iran to meet international concerns on its refusal to stop uranium enrichment and heed other U.N. Security Council demands.

"We support what the international community has said with a unified voice," she told reporters at the United Nations.

Iran's choice, she said, is to agree to measures that "would guarantee that what they're doing is solely for peaceful purposes -- and the alternative track, which is greater isolation and international pressure."

With the stakes raised by Tehran's revelation last week of a secret uranium enrichment site, a move by the U.S. to break precedent and meet directly with Iran would reflect the Obama administration's determination to get results at today's gathering.

Briefing reporters in Geneva, a senior U.S. official raised the possibility of a meeting between the Americans, represented by William Burns, the under secretary of state for political affairs, and Iran's chief negotiator, Saeed Jalili.

The official demanded anonymity because of the private nature of the talks.

Such one-on-one negotiations would "offer an opportunity to reinforce the main concerns that we'll be emphasizing in the meeting," the official said.