The United States and Europe vowed Thursday to keep up pressure on Iran to come clean about its nuclear program, saying the country's continued refusal to prove its intentions are peaceful will draw new penalties.

Secretary of State Hillary Rodham Clinton and European Union foreign policy chief Catherine Ashton said efforts to craft additional sanctions on Iran will continue. Clinton said the door to engagement remained open but stressed that the international community would not be "waited out" by Iranian resistance and would not "back down" on its demands.

"Regrettably, Iran has not responded to that engagement even as the international community's concern about the intent of Iran's nuclear program has increased," Clinton said. "We will continue our close consultation on next steps in keeping with our dual-track approach. But let me be clear, we will not be waited out, and we will not back down."

"Iran has a very clear choice between continued isolation and living up to its international obligations," she said. "We are going at this in a very concerted and unified manner because we think it is important to send that message to the Iranian leadership that the world will act, and the world will act together."

Ashton, standing beside Clinton at a State Department news conference, echoed that sentiment. "We stand together with the United States" on Iran, she said.

"We want to have dialogue," Ashton said, adding, however, that six years of talks with Iran on the issue "have not brought us to the outcome that we wish."

"So we do have to consider what else needs to be done and we stand ready to do that," Ashton said.

The U.S. and the EU are major players in a drive to persuade Iran to stop enriching uranium in exchange for incentives. Iran insists its nuclear program is for civilian energy production, but the U.S. and its allies suspect it is trying to develop atomic weapons.