OCCUPIED JERUSALEM: Israel fears that a jihadist offensive that has swept up swathes of Iraq may prompt concessions to arch-foe Iran from its longtime ally the United States.

"If Washington needs Tehran's help to solve the Iraq crisis, the United States will need to be more flexible in negotiations on Iran's nuclear programme," public radio cited a senior official as saying.

Tourism Minister Uzi Landau warned: "We're in a situation where, to confront the threat from the global jihad, we rely on Iran and its allies."

The rise of the jihadist Islamic State in Iraq and Greater Syria (ISIS), which has seized Iraq's second city Mosul and a swathe of its north and centre over the past 10 days, has prompted talk of possible cooperation between Washington and Tehran to help stop the insurgency.

A top Iranian official said on Wednesday that Tehran could consider working with the United States over the crisis in Iraq if talks on its nuclear programme are successful.

The Iranian official's comments came after US Secretary of State John Kerry said Monday he would be open to cooperating with Iran on Iraq.

"I wouldn't rule out anything that would be constructive," Kerry told Yahoo News when asked if the United States would cooperate militarily with Iran, one of Iraqi Prime Minister Nouri al-Maliki's key allies.

US Deputy Secretary of State William Burns held a brief meeting with Iranian officials in Vienna on Monday on the sidelines of talks between Tehran and the major powers over its controversial nuclear programme.

Israel -- which has the Middle East's sole if undeclared nuclear arsenal -- is deeply opposed to the talks, which also involve its US ally and aim for a long-term deal to set out clear limits to Iran's nuclear ambitions.