In recent weeks, Iran has dramatically escalated efforts to seal off its cache of near bomb-grade uranium, deliberately collapsing tunnels and booby-trapping entrances with explosive mines, according to five sources familiar with US intelligence.

Getting to the roughly half-a-ton of highly-enriched uranium is now far more difficult, dangerous and time-consuming than it already was just a month ago, when President Donald Trump was publicly signaling that he might order the US military to seize it, the sources said.

The new fortifications by the Iranians add an additional layer of complexity to the Trump administration’s proposed deal with Tehran to remove and destroy its uranium, and the move raises questions about who will take on the dangerous task of digging it out.