Western countries' sanctions against Iran are boosting rather than hurting the Iranian government, opposition leader Mehdi Karroubi said in an interview with a British newspaper published Thursday.

Karroubi said the sanctions gave President Mahmoud Ahmadinejad's government a justification for clamping down on opponents.

"Look at Cuba and North Korea," he told the Guardian newspaper. "Have sanctions brought democracy to their people?

"They have just made them more isolated and given them the opportunity to crack down on their opposition without bothering themselves about the international attention," he said in an email exchange.

Karroubi added that sanctions were "strengthening the illegitimate government" and giving them an "excuse... to suppress the opposition by blaming them for the unstable situation of the country."

The former parliament speaker has twice run unsuccessfully against Ahmadinejad.

Following the disputed presidential election in 2009, he -- along with Mir Hossein Mousavi -- emerged as leader of popular street protests which called the vote fraudulent and demanded a recount.

The UN Security Council hit Iran with a fourth set of sanctions over its disputed nuclear programme in June.

The United States and European Union have since imposed tougher measures of their own which contain provisions to penalise Tehran's trading partners.

The Guardian reported that Karroubi had also spoken out against Iran's treatment of Sakineh Mohammadi Ashtiani, sentenced to death by stoning after being convicted of adultery and conspiracy to murder her husband.