Ali Larijani's comments came before a meeting between Iranian and European Union officials in Brussels in which the deal is to be discussed.
"Our negotiations with the Europeans will be on Wednesday, but it is only the beginning of the talks," he told Iranian state television.
Larijani said the most likely date for the response would be August 6.
On Monday, Western diplomats set Iran an informal deadline to suspend uranium enrichment and agree to talks on its nuclear programme, or face the threat of United Nations sanctions.
The diplomats said Iran had until July 12 to agree to a package of economic and other incentives if it gave up uranium enrichment.
The envoys also said that Russia and China were closer to supporting the West on UN Security Council action if Tehran refused the incentives package.
It is thought that Javier Solana, the EU foreign policy chief, will urge Larijani to commit his country to suspending enrichment and starting negotiations on the package, the diplomats said.
If Iran does agree to suspend enrichment and resume talks by the July 12 deadline, foreign ministers of the five permanent Security Council nations and Germany are likely to decide to restart discussions on a Security Council resolution.
Iran says it has a right to the technology for nuclear power under the Nuclear Nonproliferation Treaty (NPT).
However, Western nations are concerned that Tehran wants to enrich uranium to weapons-grade levels for use in nuclear warheads.