Nearly one-out-of-three voters (32%) now regard Iran as the biggest threat to U.S. national security, according to a new Rasmussen Reports national telephone survey.

That’s up 10 points from a month ago, when Iran was virtually tied with North Korea on the lists of countries U.S. voters were most concerned about. Last week, the United States and its allies disclosed the existence of a secret Iranian nuclear plant and stepped up efforts to shut down that country’s nuclear program which is suspected of developing nuclear weapons.

Republicans are more suspicious of Iran than Democrats and voters not affiliated with either major party.

Just 19% of all voters now list North Korea as a bigger threat to U.S. national security, followed by China at 14%. Pakistan, Afghanistan, Iraq and Russia all rate single-digit levels of concern.

Fifty-one percent (51%) of voters say President Obama has not been aggressive enough in responding to Iran's nuclear program. In mid-August, 70% of Americans said Iran is an enemy of the United States, putting it second only to North Korea on the U.S. enemies list.

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Voters continue to express a high level of concern about the war in Afghanistan.

For the second month in a row, 55% of voters say they expect the war in Afghanistan to get worse in the next six months, up from 42% in June. The president and his advisers are currently conducting a high-profile debate over whether to send more troops to Afghanistan as the top U.S. military commander there has requested.

Just 18% say the situation in Afghanistan will get better in the next six months, while 27% expect it to remain about the same.

Seventy percent (70%) of GOP voters say the situation in Afghanistan will get worse in the next six months, compared to 45% of Democrats and 51% of unaffiliateds.

Americans are closely divided over whether the United States should send more troops to fight the war in Afghanistan. Thirty-seven percent (37%) say yes to sending more troops, while 40% say no. Most Democrats oppose sending more troops, while most Republicans feel otherwise.

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