Israeli Defence Minister Moshe Yaalon has hardened his stance on Iran following developments in nuclear talks with the West and would now back a unilateral Israeli strike, a newspaper reported on Tuesday.

Yaalon also accused Washington of "demonstrating weakness" in the Ukraine crisis in comments published by Haaretz newspaper.

Known for his hawkish views on the Middle East peace talks, Yaalon has long been regarded as a moderate on Iran, urging restraint as talk raged over a possible strike on Tehran's nuclear facilities to prevent it from acquiring military knowhow.

But the minister's attitude has changed as world powers have responded to Iran's "smile offensive" in the ongoing nuclear talks, the paper said as a fresh round of negotiations resumed in Vienna.

"We had thought the ones who should lead the (military) campaign against Iran is the United States... But at some stage, the United States entered into negotiations with them, and unhappily, when it comes to negotiating at a Persian bazaar, the Iranians were better," he said.

He charged that while the world powers know Iran "cheats," Westerners "prefer to put off confrontation, if possible to next year, or the next president".

"Therefore on this matter we have to behave as though we have nobody to look out for us but ourselves," he said, in remarks aligning him with Prime Minister Benjamin Netanyahu who has repeatedly refused to rule out a unilateral Israeli strike.

Israel and much of the West believe Iran's nuclear programme is a cover for developing a military capability, a charge denied by Tehran.

But there has been a noticeable softening towards the Islamic Republic since the election of reputed moderate President Hassan Rouhani, a move that has angered Israel.

Yaalon also accused Washington of "demonstrating weakness" in the confrontation with Moscow over Ukraine, the biggest stand off between the two powers since the Cold War.

If the US continued to demonstrate weakness overseas, its own security would be harmed, he said.

"If you sit and wait at home, terrorism will come again," Haaretz quoted him as saying.

"If your image is feebleness, it doesn't pay in the world... I hope the United States comes to its senses. If it doesn't, it will challenge the world order and the United States is the one that will suffer."