VIENNA — Secretary of State John Kerry warned Iran on Sunday that hard choices were still needed to seal a landmark nuclear accord, and that the United States was prepared to walk away if a sound agreement could not be reached.

“We are not yet where we need to be on several of the most difficult issues,” Mr. Kerry said in a statement in front of the Palais Coburg, the Vienna hotel where the talks are being held. “This negotiation could go either way.”

Mr. Kerry’s remarks came two days before a target date for wrapping up the agreement and as foreign ministers from the other world powers involved in the talks are heading here for what is intended to be the homestretch in the long-running negotiations.

Mr. Kerry said it was still possible to reach an agreement by Tuesday, which would enable the Obama administration to submit the deal to Congress this week for a 30-day review period.