"There's a great deal of concern about the lack of sincerity on the Iranians' part," Paul said Friday.
Iran's supreme leader expressed pessimism this week about the deal, saying that he neither supports nor opposes it and that all sanctions on the country must be lifted for a final agreement.
Paul voted to impose sanctions on Iran and was one of 47 senators who signed a letter to the country's leaders over the negotiations, suggesting they could later be undone by Congress. Paul has said that he is for a negotiated deal and signed the letter to help Obama "negotiate from a position of strength" by telling Iran that he has to deal with Congress.
Paul said Friday that "proof of compliance and proof of goodwill" must be established before any sanctions are removed.
Secretary of State John F. Kerry and his team are working to finalize a deal by a June 30 deadline.
Paul said Friday the support for negotiations is where "I differ from some Republicans," and said he doesn't think they need to be immediately stopped.
"I think they need to keep the sanctions in place but I think keeping the door open, continuing conversations, is better than war," he said.
Paul's campaign kicked off with a number of interviews where Paul bristled at questions about his Iran policy, including queries on a 2007 comment where he dubbed the idea that Iran posed a threat to U.S. security a "ridiculous" one. Paul has said that the comment was eight years ago and both his views -- and the world -- have changed.