UN nuclear inspectors have warned part of their surveillance of Iran's alleged nuclear weapons programme has gone dark.
The head of the UN nuclear watchdog said his inspectors are close to being unable to "guarantee" they know the size of Iran's stockpile of enriched uranium after a fruitless trip to Tehran produced no results.
Mystery surrounds a key Iranian workshop that makes parts for uranium centrifuge machines. The machines are used to enrich uranium - the vital ingredient for atom bomb-making.
Iranians said Israeli sabotage destroyed one of four cameras installed by the International Atomic Energy Agency (IAEA) to monitor activity at the sensitive site at a Karaj facility.
However, they also removed the other three cameras and said the destroyed camera's footage is 'missing'.
The IAEA said it has not been allowed to re-enter the site and install new cameras, despite Iran's agreement to let it do so two months ago.
Without cameras, the IAEA cannot be sure material and equipment are not being diverted to any secret nuclear weapons programme.
Iran insists its nuclear activities are peaceful but outside powers suspect it of working towards building the bomb.
The stark warning from the IAEA makes for a worrying backdrop to efforts to revive the 2015 Iranian nuclear deal. Talks to resuscitate it broke up in June. They begin again next week in Vienna, Austria, in what may be the last chance to save the agreement.
The deal brokered by the UK, France, Germany, the US, Russia and China, and Iran after five tortuous years of negotiations lifted sanctions on Iran in exchange for a freeze in its nuclear programme.
But the agreement was unilaterally scuppered by America under President Trump.
Iranians are desperate to have sanctions lifted to rescue their deeply troubled economy, but have broken the terms of the deal themselves.
Most worrying is Iran's return to enriching uranium, most recently to 60% levels. Under the deal, they were allowed to enrich up to 3.67%. To build a nuclear weapon they need uranium enriched at 90%.
Western diplomats hope the Iranians are acting tactically, trying to increase their leverage before the resumption of talks.
But the closer the Iranians come to building the bomb the less incentive there is for outside powers to invest the diplomatic effort required to reach a new deal or revive the last.
Iran has a new hardline regime and its true intentions may become clearer as it reveals its opening hand at negotiations next week. If outside powers remain unconvinced the talks may break up soon after they begin.
Without a new Iranian nuclear deal, the fear is of a nuclear arms race in the Middle East, or war. The closer Iran comes to a bomb, the more likely Israel is to launch a preemptive strike.
There's plenty at stake in Vienna next week and this latest warning from the IAEA doesn't raise expectations of success.