SHANGHAI - President Barack Obama made a "big mistake" in seeking to punish Iran over its nuclear program, Iranian President Mahmoud Ahmadinejad said Friday, slamming Washington but glossing over China's decision to back UN Security Council sanctions.

Ahmadinejad, visiting China's financial hub to tour the Shanghai World Expo, said the latest United Nations resolution, approved Wednesday, was "a worthless piece of paper" and would have little impact.

The effect, if any, he told reporters at a news conference at the Expo, would be to accelerate Iran's own development.

"The U.S. president thinks by accusing the nation of Iran he can undermine the role of our country in the world. I think President Obama has made a big mistake," Ahmadinejad said, accusing the U.S. of hypocrisy for leading the drive to censure his country.

Obama "came to power by the motto of change. He said he would abandon the bullying of the former administration. Now he is marching on the same path as Bush did," Ahmadinejad said.

Ahmadinejad's visit comes two days after host China yielded to international pressure to back a fourth round of nuclear sanctions targeting Iran's powerful Revolutionary Guard, ballistic missiles and nuclear-related investments in a bid to compel Tehran to co-operate with international inspectors.

The new sanctions seek to punish Iran for rejecting proposals to halt uranium enrichment and take its nuclear fuel from abroad. The West and its allies fear Iran is developing nuclear weapons, though Iran says it is seeking nuclear power only for peaceful energy and medical research purposes.

As an ally of Iran and permanent member of the Security Council, China could have exercised its veto power to block the sanctions. China's Foreign Ministry said Thursday that its support should not block efforts to coax Iran back into negotiations.

But Ahmadinejad said such talks could happen only in a "friendly atmosphere."

"Having dialogue under a hostile atmosphere has no meaning," he said.

Ahmadinejad skipped Thursday's summit in Uzbekistan of the Shanghai Cooperation Organization, which was attended by Chinese President Hu Jintao and was not scheduled to meet Chinese leaders while in China.

But the Iranian leader deflected suggestions that China's about-face on the sanctions issue might drive a rift between the two countries.

Beijing was apparently satisfied that the sanctions would not harm its economic ties with Iran, with whom bilateral trade reached at least $36.5 billion last year. China relies on Iran for 11 per cent of its energy needs and Chinese companies have major investments in Iranian energy extraction projects and the construction of roads, bridges and power plants.

"We have very good relations with China and we have no reason to weaken our relations with China," Ahmadinejad said. "We are confronting the United States. The main problem is the U.S. administration. We have no problem with others."

He accused the U.S. and other nuclear powers of intimidating other Security Council members into siding against Iran, and lashed out at the West for supporting Israel.

"The Zionist regime is doomed," said Ahmadinejad, who has repeatedly called for Israel's destruction and denied the Holocaust.

The Iranian leader insisted that efforts to stop his country from enriching uranium were unjustified and aimed at preventing it and other developing countries from acquiring needed nuclear power technology.

"The nuclear issue is just a pretext. The government of the U.S. is going to swallow the entire Middle East region. Under that context, they are going to control the whole world," he said. "I should say Iran will never allow the United States to do so."

"Even if they tear out our throats it will not happen," he said.

Ahmadinejad returned repeatedly to the issue of Obama's role in the sanctions issue, saying the move was an insult.

"I think Mr. Obama does not know the world very well," he said. "We will never be shaken by people like President Obama."