PARIS -- Iranian President Mahmoud Ahmadinejad says that those responsible for last week's massacre of more than 100 people in Syria should be punished, in unusually harsh criticism from a staunch ally.

U.N. investigators have blamed pro-regime gunmen for at least some of the carnage in Houla, saying men in civilian clothes gunned down people in the streets and stabbed women and children in their homes. The Syrian government denied its troops were behind the killings and blamed "armed terrorists."

Ahmadinejad declined to say in an interview Wednesday with the France 24 television station who he believed was behind the attack, but added: "It seems unbelievable to me that a government would engage in killing its own people .... (but) I'm not excluding anyone from this responsibility," he said.

"We (in Iran) are quite disappointed about this," Ahmadinejad said from Tehran through a translator. "Any individual who committed these murders is guilty ... The people responsible for this massacre must be punished, must be sanctioned."

The United States and Western nations expelled Syrian diplomats in protest -- a move Syria's state-run media denounced Wednesday as "unprecedented hysteria."

Ahmadinejad said the West "should not exploit" the crisis, adding that because it is clear Western governments are opposed to Syrian leader Bashar Assad, "we cannot trust what they say."

Separately, the Iranian leader insisted that the West and Iran must both "work hand in hand" in negotiations over its nuclear program, saying "we fully expect to see a change in ... approach and a constructive agreement."

The West is concerned that Iran is enriching uranium to more than 20 per cent, which could quickly be turned into nuclear weapons-grade material, but Iran maintains it won't back down and insists it is for peaceful purposes.

Iran's objected last week at a meeting with world powers in Baghdad to a proposal to suspend 20 per cent enrichment in exchange for a U.S.-supported package that would include supplying Iran with radioactive material and civilian plane spare parts. Iran seeks to have a Western oil embargo and banking sanctions eased before considering suspending enrichment.

Negotiators must explain why "what they are willing to give the Iranian people in exchange," he said.