"The timetable drawn up by other people has no influence on what we do," he said.
Mr Larijani said Tehran had been working seriously on the offer made on June 6 by six western powers, France, Britain, Germany, the United States, Russia and China, but "needed to devote sufficient time (to it)".
"We view this proposition in a positive light but there are doubts about it which we have to clarify through dialogue," he said.
"I see no reason to be pessimistic," he continued, adding, "we do not need to be given a time limit to respond ... that does not make sense. If negotiation is to take place it needs to be in the language of negotiation.”
Iran is facing mounting international pressure to show ahead of the Group of Eight (G8) summit on July 15-17 that it is ready to accept the offer from the six powers.
On Thursday the European Union, Russia and the United Nations' nuclear watchdog, the IAEA, all reiterated appeals for Tehran to respond soon, and the United States again threatened UN Security Council sanctions.
Mr Larijani held discussions in Madrid with Spanish Prime Minister Jose Luis Rodriguez Zapatero and Foreign Minister Miguel Angel Moratinos yesterday.
The latter stressed that Spain supported the international offer and hoped Iran would "respond promptly" to the proposal, which Madrid supported, according to the foreign ministry.
But Iran, which rejects Western suspicions that it is trying to covertly build an atomic bomb behind the screen of a civilian nuclear energy programme, is refusing categorically to suspend its uranium enrichment activities.
Iranian President Mahmoud Ahmadinejad has said Tehran will give its formal response to the offer in August.
Meanwhile US President George W. Bush said he is trying to get some US partners in diplomacy towards Iran and North Korea to see past their "economic interests" and take a harder line.
He did not name names, but he had been asked about opposition from Russia and China to imposing sanctions on North Korea after Pyongyang fired a barrage of missiles into the Sea of Japan this week.
"Some nations are more comfortable with sanctions than other nations. And part of the issue we face in some of these countries is that they've got economic interests," Mr Bush said.
"And part of our objective is to make sure that national security interests, security-of-the-world interests, trump economic interests. And sometimes that takes a while to get people focused in the right direction," he said.