Israel's prime minister says Iranians "deserve better" than their current government and that their lives could get worse if it gains nuclear weapons.
In an interview with BBC Persian, Benjamin Netanyahu warned: "If they get nuclear weapons this brutal regime will be immortal, like North Korea."
He also said the new President, Hassan Rouhani, could not "change the real decisions" made by the Supreme Leader.
Ayatollah Ali Khamenei and Mr Rouhani have said they reject nuclear weapons.
They insist Iran's nuclear programme is solely for peaceful purposes, but world powers suspect they are not being truthful.
There has nevertheless been a recent thaw in relations between Iran and the United States, with Mr Rouhani and President Barack Obama recently discussing the issue - the first top-level conversation between the two countries for more than 30 years.
On Tuesday, Iranian diplomats at the United Nations reacted angrily after Mr Netanyahu described their president as a "wolf in sheep's clothing" in a speech to the General Assembly.
In his interview - his first with an international Persian-language media organisation - the Israeli leader said Mr Rouhani did not "represent the Iranian people".
"He represents a desire for change, but it wasn't expressed in a free open election."
"I don't think he has the mandate to change the real decisions that are made by Khamenei. Khamenei wants nuclear weapons for Iran."
Mr Rouhani has said he has "full power and complete authority" to strike a nuclear deal. However, ultimate power lies with the Supreme Leader.
Mr Netanyahu told BBC Persian that people, himself included, wanted to see "a diplomatic solution to end Iran's quest for nuclear weapons".
"But I want to see a real solution. Not a fake one."
That would involve the "complete dismantling of Iran's ability to make nuclear weapons", he added.
"If they want civilian energy, they don't need enrichment. Uranium enrichment is necessary only if you want nuclear weapons."
"If they get nuclear weapons this brutal regime will be immortal, like North Korea. It will go on repressing the people of Iran, who deserve better. They are a great people."
The UN Security Council has adopted a series of resolutions requiring Iran to stop enriching uranium, which can be used for civilian nuclear purposes, but also to build nuclear bombs.
But Iran says it is simply doing what it has a "right" to do under the Nuclear Non-Proliferation Treaty (NPT) and is enriching uranium for power station fuel or other peaceful purposes.