(Reuters) - The United States must do more to show Iran it is serious about curtailing its nuclear ambitions because the current pressure is not working, Israel´s vice prime minister said on Thursday.

Moshe Yaalon also fired a warning at the armed Hezbollah movement in neighboring Lebanon, saying the Shi´ite group would be crushed if it tried to attack Israel at the behest of Tehran in any future war.

Speaking just days before U.S. Secretary of State Hillary Clinton visits Jerusalem, Yaalon said Iran appeared to believe it was safe from military attack or from further sanctions before November´s U.S. presidential election.

"As long they perceive this, they don´t feel like they are under pressure. That is a pity," said Yaalon, who is also Israel´s minister of strategic affairs.

"It is up to the United States to try to change this perception in Iran. This is crucial," he told Reuters, adding that Iran would top the agenda during Clinton´s visit on July 15 and 16 -- her first trip here in almost two years.

Western powers believe Iran is developing technology to build nuclear weapons and have imposed an increasingly tough regime of economic sanctions to make it reverse course.

Iran insists its atomic program is peaceful and has shrugged off the latest round of sanctions, with a European Union embargo on Iranian crude oil taking full effect on July 1.

"We´ve witnessed the impact of the sanctions in Iran, but up until now the regime prefers to suffer rather than give up its military nuclear capabilities," Yaalon said, adding that the time had come to introduce "really crippling sanctions".

Reputed to have the Middle East´s only nuclear arsenal, Israel has threatened to resort to force if it deems diplomatic and economic means are failing. Yaalon repeated the threat, but made clear that he thought Washington should lead the way.

"We believe of course that the military option should be the last resort and we believe that someone else should be doing the job. But we should be ready to defend ourselves by ourselves."

ASSAD DOOMED

A former chief of staff in the Israeli army, Yaalon said he was sure Hezbollah would jump to Iran´s aid if hostilities broke out, but predicted it would soon regret any attack.

"Any provocation will be responded to by us, by charging them with such a heavy price that they will ask for a ceasefire," he said, sitting in the sun-filled gardens of the King David Hotel in central Jerusalem.

Some Israeli officials have worried that Hezbollah, which is estimated to have thousands of missiles ready to rain across the border, might start trouble with Israel to divert attention from the woes besetting its ally, Syrian President Bashar al-Assad.

Yaalon did not see such a threat and thought that Assad was doomed to lose the violent power struggle gripping Syria.

"We are not sure when it will happen, but it will happen. From day to day we have witnessed an acceleration of the process as a result of the power that the opposition has succeeded in gaining," he said.

A junior Israeli minister said on Thursday that some Israelis were on Syria´s borders with Turkey and Jordan looking to get humanitarian aid to Syrians caught up in the fighting and to evacuate some of those trapped by the violence.

"We are sending, as volunteers, many people ... to help in the borders in Jordan and Turkey," said Ayoob Kara, a deputy minister and a member of Israel´s minority Druze community, which has close ties with fellow Druze in Syria.

"We sent food and clothes. This is humanitarian aid. We could not be apathetic when every day Assad killed one or two hundred people," he told Reuters, adding that he received messages every day from Syrians seeking help. (Editing by Mark Trevelyan) (© Thomson Reuters 2012. 07/12/12)