Sixty-two percent of Americans say they are "very concerned" about Iran's nuclear program, according to a new Pew Research Center study released Tuesday. Another 23 percent said they were "somewhat concerned," meaning 85 percent in total have at least some worries about Iran, compared to just 5 percent who say they were "not at all" concerned.

The survey results arrived just as a historic agreement was reached early Tuesday morning between Iran and six world powers to scale back its nuclear program. In a statement to the press, Secretary of State John Kerry said the deal stipulates Iran cannot achieve highly enriched uranium or weapons-grade plutonium for at least the next 15 years. Kerry, who led the U.S. effort in the final stages of the negotiations, also stressed "thorough and extensive transparency and verification measures" will assure that no terms of the agreement are breached without the United States knowing about it.

But the deal has drawn a great deal of criticism from Republicans and even some Democrats, who are concerned that too much was conceded to the Iranians, like the lifting of a conventional arms ban. Some say the deal still leaves much of Iran's capacity intact, an includes loopholes that could help Iran evade full inspections.

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As a result, the Obama administration's deal faces staunch opposition in Congress, and the president has threatened to veto any attempt at thwarting it.

Sen. Lindsey Graham R-S.C., a 2016 presidential candidate, said he is concerned that the deal will lead to a nuclear arms race in the Middle East. "This is a terrible deal. Anybody could have done better. You have taken a can of gasoline and thrown it on a fire," he added.

The Pew study noted the ideological divide on the issue. Seventy-six percent of Republicans were "very concerned" about the threat of Iran's nuclear program, in contrast to 54 percent of Democrats.

Older Americans are more worried about Iran's nuclear capability than younger generations. Seventy-three percent of participants in the 50-to-64 and 65-plus age groups said they were "very concerned," while only 40 percent of young people (ages 18 to 29) said the same.

Pew conducted the survey in 40 countries between 45,435 respondents from March 25 to May 27. According to its results, Israelis rate the threat from Iran as the single greatest global threat, and 53 percent of those surveyed said they were "very concerned" about the country's nuclear program. In the past, Iranian leaders have repeatedly called for the destruction of Israel, resulting in harsh rhetoric from Israeli leaders about any chance Iran may get to develop nuclear weapons.

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In a public address to his country, Israeli Prime Minister Benjamin Netanyahu called the deal struck with Iran a "stunning, historic mistake."

Spain was the only other country included in the survey in which more than half of its participants were very concerned about the Iran nuclear program, coming in at 52 percent.

Americans' concern about Iran's nuclear capabilities is second only to their fear about the threat of the Islamic State of Iraq and Syria. Sixty-eight percent said they were "very concerned" about the Islamic State and 21 percent said "somewhat."