His comments came as he was grilled by US lawmakers over the talks between Iran and the so-called P5+1 group, which are due to start up again in Vienna on Monday.
Under a six-month interim deal, Iran agreed to freeze its uranium enrichment program in return for sanctions relief worth some USD 6-7 billion, including the transfer of some USD 4.2 billion in frozen overseas funds.
Kerry insisted during a hearing of the Senate appropriations committee that Washington was testing whether the new Iranian leadership of President Hassan Rouhani was serious in reining in its nuclear program, which the West believes is a covert grab for an atomic bomb.
"There's some very tough decisions the Iranians are going to have to make -- very tough -- in order to meet the international community's standard for certainty as to the peacefulness of this program."
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But he hit back against criticism that Iran was now attracting cash-flow and business back to the country, and that the sanctions relief was being used to fund Hezbollah militants fighting in Syria.
"We have made it crystal clear that Iran is not open for business. They have accepted that," Kerry said.
"They are not cutting deals. There are people who have traveled, but there have not been new deals."
He stressed again that the tough sanctions regime put in place against Iran, which has brought its economy to its knees, had not been dismantled.
"Nothing in the architecture of the sanctions regime has been changed whatsoever. Iran's economy contracted by 6 percent last year. It is expected to contract again this year. Inflation remains at almost 40 percent," Kerry told lawmakers.