US President George W Bush, who is visiting Germany, has said the dispute over Iran's nuclear programme can still be resolved diplomatically.
Mr Bush says a diplomatic solution will be "hard work"
Mr Bush said it would be dangerous if Iran had nuclear weapons and said the world was united in opposing this.
Iranian President Mahmoud Ahmadinejad warned that his country would not give up its right to nuclear technology.
On Wednesday, six major powers referred Iran back to the UN Security Council for possible action over the dispute.
A package of incentives put forward by the six - US, Russia, China, Britain, France and Germany - is still being considered by Iran despite pressure for a quick response.
Mr Ahmadinejad said that while the proposals were a positive step forward, it would take time to solve the crisis, and he reiterated that Iran would not respond to the package until the middle of August.
However, Mr Bush said: "I truly think they're trying to wait us out, that it's only a matter of time before people lose their nerve. And I think they are going to be sorely mistaken.
"The Iranians must understand that they can't wait us out and can't hope to split a coalition - and so the first step is to go to the United Nations and speak with as common a voice as possible," he said.
German Chancellor Angela Merkel said Iran still had not given a firm response to proposals for addressing Iran's energy concerns if it stopped enriching uranium, and a new course might be necessary.
"If Iran does not agree to this offer, then unfortunately we need to go down other routes," she said.
However, Mr Ahmadinejad warned Tehran could review its co-operation with the UN nuclear watchdog, the IAEA, and with the Non-Proliferation Treaty (NPT).
Mr Ahmadinejad says his country will not be pressured
"Up to now the Iranian people have acted within the framework of the NPT and the IAEA, but if they reach the conclusion that Western countries do not have goodwill and sincerity...they (the Iranian people) will revise their policy," the website of Iranian state television quoted the president as saying.
Mr Ahmadinejad's comments came the day after his country was referred back to the UN Security Council by foreign ministers from the five permanent Security Council members and Germany for possible punishment over its nuclear programme.
French Foreign Minister Philippe Douste-Blazy said they would seek a resolution threatening sanctions, if Iran did not agree by mid August to stop enriching uranium.
Iran says its nuclear programme is peaceful, but the US and some other countries say it is trying to produce nuclear weapons.
The six powers want a pledge that Iran will abandon uranium enrichment in exchange for the incentives.