"I would say that the Iranian insistence on not discussing ballistic missiles is a big, big problem," Rubio told reporters on the eve of new talks between the United States and Iran.

Rubio, however, steered clear of describing if the talks in Geneva would be a make-or-break moment in deciding if the United States attacks Iran.

"The president wants diplomatic solutions. He prefers them greatly," Rubio said on a visit to the small Caribbean nation of Saint Kitts and Nevis.

As for the talks, "hopefully they're productive, but eventually we'll have to have conversations about more than just a nuclear program."

On a potential strike on Iran, Rubio said: "The president's made no decision on that, so I don't know if Thursday's the key date for that. I think progress needs to be made."