By Francois Murphy

VIENNA (Reuters) -U.N. nuclear watchdog chief Rafael Grossi on Thursday identified Isfahan, home to one of Iran's biggest nuclear facilities, as the location of a uranium enrichment plant that Iran said it would soon open in retaliation for a diplomatic push against it.

The day before Israel launched its military strikes against Iranian targets including nuclear facilities last Friday, Iran announced it had built a new uranium enrichment facility, which it would soon equip and bring online. Tehran did not provide details such as the plant's location.

Iran's announcement was part of its retaliation against a resolution passed by the International Atomic Energy Agency's 35-nation Board of Governors declaring Tehran in breach of its non-proliferation obligations over issues including its failure to credibly explain uranium traces found at undeclared sites.

Had it gone online, the new enrichment plant would have been the fourth in operation in Iran. But Israel's attacks on Iranian nuclear facilities destroyed one of those plants and put another out of action by killing its power supply, the IAEA has said.

"There was an announcement, quite coincidentally, on the eve of the start of the military operation by Israel of a new enrichment facility in Isfahan, precisely, that we were going to be inspecting immediately, but this inspection had to be postponed, we hope, because of the start of the military operation," Grossi said.

He did not say where exactly in Isfahan the planned plant was, but he said the nuclear complex there is "huge".

The IAEA has previously reported that Israeli military strikes on Friday damaged four buildings at Isfahan, including the Uranium Conversion Facility that transforms "yellowcake" uranium into the uranium hexafluoride feedstock for centrifuges so that it can be enriched.

Grossi told the BBC on Monday that the "underground spaces" at Isfahan did not seem to have been affected. Officials say those spaces are also where much of Iran's most highly enriched uranium stock has been stored.

The IAEA has not, however, been able to carry out any inspections since the strikes.

(Reporting by Francois Murphy; Editing by David Gregorio)