Iranian President Mahmoud Ahmadinejad accused Western nations on Wednesday of hypocrisy and said their expressions of concern over nuclear programmes were a "big lie".
The Iranian leader was speaking on a visit to Indonesia, which said Tehran had been receptive to its offer to help mediate the Islamic republic's dispute with critics of its nuclear project.
"I'll tell you, they are not concerned with nuclear programmes ... They are themselves engaged in nuclear activities and they are expanding day by day. They test new brands of weapons of mass destruction every day," Ahmadinejad told a news conference after meeting Indonesian President Susilo Bambang Yudhoyono.
"Big powers pretend (they) are concerned, but it's a big lie," the Iranian leader said.
Iran is under pressure to rein in a nuclear programme it says is for peaceful purposes but some countries fear is really aimed at developing weapons.
Indonesia, the world's most populous Muslim country, has offered to help mediate on the issue, Yudhoyono and his spokesman said.
"We can cooperate well in reducing the tension and move toward continuing talks and negotiations," Yudhoyono told reporters.
Spokesman Dino Patti Djalal said "Iran was very receptive" to Indonesia's offer to help mediate.
"We hope it will happen. We need to breathe new life into negotiations," he said.
Speaking of a letter sent to US President George W. Bush, which Washington shrugged off as an attempt to divert attention from the nuclear issue, Ahmadinejad said sending it was the right decision and that he had no comment on the US reaction.
The United States has pushed for international action on the issue, and with a group of nations including China and Russia has authorised Britain, France and Germany to work on a package of carrots and sticks to entice Iran to change its programme.
President Bush received the 18-page letter from Ahmadinejad on Monday, the first publicly announced personal communication from an Iranian president to his US counterpart since the 1979 Islamic revolution.
Despite Washington's initial cool reaction, analysts say the letter might buy Tehran more time to pursue its programme and improve its standing as a regional leader.
Indonesian Vice President Jusuf Kalla met Ahmadinejad on Wednesday and told reporters that "Indonesia will take a role behind the scenes for peaceful purposes on Iran issues".
He also said without elaborating that Indonesia supported nuclear programmes for peaceful use.
Kalla plays an influential role in Yudhoyono's administration and helped broker a successful peace deal last year in Aceh province, where a separatist rebellion had simmered for decades.
Despite the attention to the nuclear issue, the prime stated purpose of Ahmadinejad's visit to Jakarta is not the nuclear issue but development of economic ties.
Iran is in the process of investing several billion dollars in the oil and gas sector of fellow Organisation of the Petroleum Exporting Countries (Opec) member Indonesia and both countries are eager to boost trade.
On Friday, Ahmadinejad is due to fly to Bali for a meeting of the Developing Eight group that also includes Indonesia, Nigeria, Malaysia, Egypt, Turkey, Pakistan and Bangladesh.
Economic development, including peaceful uses of nuclear energy, figure high on the agenda for the meeting, which will end on Saturday.