The head of the UN nuclear agency has "serious concerns" that Iran may be hiding secret atomic weapons work and admits he failed in his latest attempt to unearth them.

International Atomic Energy Agency chief Yukiya Amano spoke to the 35-nation IAEA board amid backroom diplomacy aimed at coming up with substantial pressure on Iran to end its defiance and address concerns of its nuclear activities.

The conference opened as fears grow that Israel may soon strike Iran in an attempt to destroy its nuclear facilities.

Mr Amano summarised the most pressing worries – Tehran's rebuff of two recent attempts to probe the weapons programme and a sharp, recent increase in uranium enrichment, which Iran says it needs for nuclear energy, but which can also produce weapons material.

The US and its Western allies are lobbying Russia and China to back a resolution critical of Iran's refusal to heed IAEA and UN Security Council demands that it give full nuclear transparency.

Moscow and Beijing are traditionally brakes on Western efforts to tighten sanctions on Iran.

Any resolution passed by the IAEA goes to the UN Security Council and could be used as a platform for additional sanctions on the Islamic Republic, which already is the focus of four sets of UN sanctions meant primarily to pressure it to give up enrichment. The US, the EU and others have also slapped Tehran recently with financial and economic penalties aimed at its banking system and oil export industry.

Recent moves to boost uranium enrichment at Fordo, an underground facility that may be able to withstand aerial attack, are of particular concern.

In a report last month, Mr Amano said Tehran had increased monthly enrichment to 20% at Fordo in the last four months, and expanded lower-level enrichment at another facility.

Both lower enriched uranium below 5% and 20% enriched material can be processed further to 90% – the level used to arm nuclear warheads. But 20% enrichment is of particular concern because it can be turned into weapons-grade material much more quickly and easily.

Two weeks ago IAEA experts returned from Tehran after their second failed attempt in a month to persuade Iran to end nearly four years of stonewalling on what the agency says is growing intelligence-based information that Iran has worked – and may still be working – on a nuclear weapons programme.

"We have some indication that activities are ongoing at the Parchin site," Mr Amano told reporters.