US will back new Iran sanctions

She spoke as the UN nuclear agency said it could offer no "credible assurances" that Iran was not building a bomb.

UN Security Council members the US, UK, China, France and Russia meet on Monday in Washington to mull their next step.

But Iran's top nuclear negotiator said the UN report backed Tehran's claim its nuclear programme was peaceful.

Tehran refuses to stop enriching uranium, insisting its work is aimed purely at generating electricity.

Travel bans

The International Atomic Energy Agency (IAEA) report did praise Iran for granting its inspectors access to previously off-limits sites, but said it remained evasive on key issues.

The nuclear watchdog struck a deal last August with Iran on a timetable to resolve questions over aspects of its past nuclear activities.

But Friday's report said Tehran had evaded a proper response to claims it had made secret efforts to "weaponise" nuclear material.

It had also ducked questions about alleged high explosives testing and design work on a missile warhead, the IAEA found, noting: "This is a matter of serious concern."

Iran was also still openly enriching uranium in defiance of UN resolutions and testing advanced centrifuges to speed up the process, said the inspectors.

The US secretary of state said: "This report demonstrates that whatever the Iranians may be doing to try to clean up some elements of the past, it is inadequate given their current activities.

"What we all have to worry about... is the future in which Iran could start to perfect the technologies that could lead to a nuclear weapon."

Iran's chief nuclear negotiator Saeed Jalili told reporters: "This report showed that our activities are peaceful."

Iran's key nuclear sites

But BBC diplomatic correspondent Jonathan Marcus says the IAEA findings are definitely not a clean bill of health.

On Thursday, Britain and France introduced a UN Security Council resolution - a draft of which was approved last month - with support from the US, Russia, China and Germany.

It seeks to expand the number of Iranian companies targeted by sanctions and impose travel bans on certain Iranian officials.

A declassified US intelligence report last December judged that the Iranians had put a nuclear weapons program on hold in 2003.

But the US, Israel and others contend Iran's continued advances in the crucial centrifuge work will eventually give it a capability to build a bomb.