He did not specify what changes to the package Iran might seek but Tehran has always rejected the central crux of the proposal – that it should give up enriching uranium. Without a concession on this, the deal will fail.
"We should study the package offered. We should classify it. There are points which are acceptable. There are points which are ambiguous and there are points that we believe should not exist," Foreign Ministry spokesman Hamid Reza Asefi said.
"We have received the package, we should give our views in response to that package and we will offer our proposals," he told a weekly news conference.
Although Asefi's remarks are a strong indication Iran is unhappy with the terms offered, the Foreign Ministry does not have the last word on nuclear issues.
Supreme Leader Ayatollah Ali Khamenei has entrusted nuclear matters to the Supreme National Security Council and appointed Ali Larijani as chief negotiator.
'We welcome any negotiations with no preconditions'
Iran has been referred to the UN Security Council, where it could face sanctions, after failing to convince the international community that its atomic scientists are only looking to build power stations and not weapons.
The United States, France, Germany, Britain, Russia and China have agreed a set of incentives for Iran on the condition that it stops making nuclear fuel, something Tehran has repeatedly said it will never do.
Among the incentives, which Western diplomats say include offers of a light-water reactor and a facility for storing atomic fuel, is a very rare US offer to join the European Union's direct talks with Iran.
Although President Bush has said Iran should reply within weeks rather than months, Asefi said Iran would not be rushed.
"We have not been given a deadline ... but that does not mean we are seeking to buy time," he said.
When asked about the part of the offer concerning direct talks with the United States, Asefi said: "We welcome any negotiations that do not have preconditions."
In Iranian rhetoric "preconditions" normally refer to the Western insistence that Tehran drop its atomic fuel work.