"This (offer) contains positive points, such as nuclear reactors for Iran," Iran's nuclear negotiator, Ali Larijani, told a news conference in Cairo. "At the same time there are problems and ambiguous points."
Larijani said that Iran would not be threatened but that it was prepared to negotiate.
"There were some ambiguous points like the uranium enrichment in Iran. The subject has not been clarified in a transparent way," he said. "We hope there will be constructive discussions."
Larijani said the offer from the US, France, Germany, Britain, Russia and China included a proposal for regional stability talks in which, he said, Iran were ready to take part.
"We are ready to give this incentive for the sake of security and stability in the region because Iran is a regional force," he said.
Larijani was in Cairo for meetings with Hosni Mubarak, the Egyptian president, and the Arab League, where he sought to reassure Arab nations over Tehran's nuclear ambitions.
"The Iranian nuclear programme does not represent any danger to the Arab and Islamic nations because it deals with peaceful nuclear technology," he said after talks with Amr Moussa, the Arab League secretary general. "We don't seek a nuclear bomb and the programme will help Arab and Islamic countries."
Moussa said that Larijani presented "full assurance that their program is peaceful" and that he discussed with him concerns among Gulf Arab countries over the programme.
Hamid Reza Asefi, a foreign ministry spokesman, earlier said that Iran was still examining the proposals, but he insisted that Iran was not stalling.
"We started studying the package the moment it was presented to us," Asefi said.
George W Bush, the US president, said on Friday that Iran would have weeks, not months, to decide whether to accept the proposals or face the prospect of penalties.
Asefi also stated that Iran would not give up its nuclear "rights".
"We are definitely not going to compromise on our rights," he said. "We are going to act according to our responsibilities and rights. We are holding the initiative."
The package put forward at the UN is aimed at restarting negotiations with Iran over its nuclear programme.
It included concessions by the US aimed at getting Tehran to freeze uranium enrichment. The US would provide Iran with peaceful nuclear technology, lift some sanctions and join direct negotiations.
When presented with the proposal's details on Tuesday, Iran said they contain "positive steps" and ambiguities, which it said had to be cleared up in further talks. It said it would study the package before announcing its stance.
The package contains the threat of UN sanctions if Iran remains defiant.