But it does seem to be escalating. What Tehran is trying to do now, most analysts believe, is in part further retaliation. But its rulers may also be indicating that the Islamic Republic now has a range of new and potentially damaging options in reserve should its nuclear facilities be bombed.

SIGNALLING THROUGH COVERT ACTION?

The penetration of Israeli airspace by an unmanned drone apparently operated by Lebanese militant group Hezbollah -- a long-term Iranian ally -- was, perhaps, one of the clearest examples so far. The drone was shot down by Israel’s military in the vicinity of its main nuclear facility at Dimona.

Iran has long been believed to be putting resources into a drone program and may have gathered useful tips after a classified U.S. Sentinal stealth drone came down in the country last year. While the Hezbollah drone was unarmed, an attack with multiple drones laden with explosives might prove harder to stop.

The dramatic spike in suspected Iranian cyber attacks this year also has some in the U.S. distinctly worried. While direct denial of service attacks on U.S. banks -- widely seen as retaliation for US sanctions and attempts to freeze Iran from the international financial system -- were seen as relatively simplistic, attacks on US allies in the Gulf were more complex.

The most worrying, experts say, were those on Saudi oil firm Aramco and Qatari gas export facilities. Last month, US Defense Secretary Leon Panetta described the Saudi attack as the most sophisticated yet launched on a private company, effectively destroying tens of thousands of computers -- although he stopped short of blaming Tehran directly.

Iranian officials have tended to deny involvement. But they say they have continued to come under cyber attack themselves with systems at Iran’s own oil facilities, communications and infrastructure firms suffering problems last month.

“The problem is that these are secret forms of warfare that are rarely, if ever, discussed publicly,” a veteran former CIA official and now a senior fellow at the Brookings Institute told an event last month. “And yet the implications could be colossal. What do we do, for example, if it turns out the Iranians can shut down the entire Saudi oil production.”

In the absence of direct face-to-face negotiations, such actions can also be a diplomatic tool in their own right.