Isfahan, Iran ? In the end, there was nothing U.N. inspectors could do but watch as Iranian technicians broke open seals on the doors to a unit in Iran’s nuclear program and fired up equipment that the West had hoped would remain mothballed forever.

European and American diplomats were faced with the same lack of options, trying to figure out how to stop Iran when it is determined to move ahead with a program the West fears will produce nuclear weapons.

The breaking of the seals Wednesday at the Isfahan Uranium Conversion Facility was the latest step in Iranian brinkmanship over its nuclear ambitions, which it says are peaceful, aiming only to produce electricity.

Iran rejected European proposals for limiting its program in return for economic incentives and shrugged off threats of U.N. sanctions. Europe and the United States backed down from that threat – in part because it could cause a backlash prompting Iran to harden its position further, and because it’s unclear sanctions would even pass the U.N. Security Council.

Instead, diplomats at the U.N. watchdog agency – the International Atomic Energy Agency – were debating how strongly to rebuke Iran for reopening the plant.

And Washington and Europe were placing their hopes in negotiations that Britain, Germany and France have been holding with Tehran. Iranian President Mahmoud Ahmadinejad said Tuesday he would make new proposals soon.

But the resumption of operations at the Isfahan plant were a sharp signal to Europe that Iran is serious in saying it won’t give up its program to process uranium into nuclear fuel. The West fears that same technology will be used to develop weapons.

Iran had agreed to freeze most of its nuclear program in November as a gesture in the negotiations. So far, it says it won’t resume the most crucial part of the process – uranium enrichment, which can produce either fuel or weapons. Reopening its enrichment plant in the town of Natanz would likely spark a stronger reaction from the West.

The Isfahan facility carries out an earlier step in the process, converting yellowcake – raw uranium – into uranium hexaflouride gas, UF-6, the feedstock that in the next stage is fed into centrifuges for enrichment.

Iran notified the IAEA Tuesday that it wanted the Isfahan seals removed, saying either the agency could do it, or Iranian officials would remove them under IAEA supervision. Iran voluntarily allowed the seals to be put in place and so has the right to have them taken away.

Grudgingly, the agency authorized the Iranians on Wednesday to break the seals. “This should (in) no way be seen as an endorsement,” IAEA spokeswoman Melissa Fleming in Vienna said.