There is "no guarantee that the Americans, after the signing of the additional protocol and inspections of nuclear installations, will not invent other pretexts to accuse Iran of developing weapons of mass destruction," said Ali Larijani, the head of Iranian radio and television, quoted by the state IRNA news agency.
Washington and the European Union have pressed Iran to sign the additional protocol, but the issue has sparked a heated debate in Iran amid US suspicions the country is using its nuclear energy program as a cover to develop weapons.
"There is no reason to accept signing the additional protocol because they (NPT members) did not help the Islamic republic of Iran to develop nuclear technology," added Larijani, who is appointed directly by Supreme leader Ayatollah Ali Khamenei.
If "Tehran resists the pressures, the Westerners will end up changing their stance towards Iran," he concluded.
While some Iranian moderates have expressed support for signing the additional protocol, several hardliners have said it would open the door to excessive interference in the country's domestic affairs.
Experts from the International Atomic Energy Agency (IAEA) visited Iran early this month for talks to address Tehran's uncertainties over the protocol. On Wednesday Iran's atomic energy chief said the two sides should reach "positive" results by September.
The Iranian nuclear case will be reviewed by the IAEA's board of governors on September 8, with the possibility that the case might be forwarded to the UN Security Council.