WASHINGTON -- Secretary of State Hillary Rodham Clinton said Friday that Iran's refusal to prove that its nuclear intentions are peaceful has "profound consequences" for world security.

Clinton said the Obama administration's offer to engage with Iran remains on the table, but Tehran faces new penalties and greater isolation unless it complies with demands that it be forthcoming about its plans.

She also said that time is running out for the country to show it is serious about addressing concerns about its nuclear program and that the matter would be a top priority at next week's United Nations General Assembly session.

"Iran's continued failure to live up to its obligations carries profound consequences for the security of the United States and our allies," Clinton said in a speech at the Brookings Institution that previewed the administration's agenda for the U.N. meeting. "Our concern is not Iran's right to develop peaceful nuclear energy, but its responsibility to demonstrate that its program is intended exclusively for peaceful purposes."

At the United Nations, Clinton will meet with her counterparts from the other permanent members of the U.N. Security Council -- Britain, China, France and Russia -- along with Germany -- to plot a way forward on Iran. The six nations have offered Iran incentives to halt activities that could lead to development of atomic weapons.

"Our message will be clear: we are serious and we will soon see if the Iranians are serious," Clinton said, adding that the U.S. will work with its partners to keep up the pressure on Iran.

Iran, which insists its nuclear program is peaceful, has yet to accept the package of incentives despite warnings that its refusal to do so will lead to another round of U.N. sanctions. Iran is already under three sets of Security Council sanctions.

The meeting at the U.N. is set for next Wednesday. Shortly afterward, on Oct. 1, top diplomats from the six countries will meet Iran's chief nuclear negotiator to gauge the country's response.

The flurry of activity comes after the release of a new report from the U.N. nuclear watchdog that said Iran has still not addressed questions about the nature of its program and the revelation that experts at the International Atomic Energy Agency believe it has the ability to make a nuclear bomb and is working on developing a missile system to deliver it.

U.S. officials have long made that claim but it has not neccessarily been shared by others, including China and Russia, which have balked at imposing new sanctions on Iran.

Even with the new nuclear findings, a fierce Iranian government crackdown on the opposition after elections and President Mahmoud Ahmadinejad's latest questioning of the Holocaust and lashing out at Israel on Friday, Clinton said the administration offer of engagement still stood.