The Iranian position came after Europe increased pressure on Tehran to maintain its suspension of a programme that officials fear could result in the production of a nuclear bomb.
"No certain day is fixed for resumption of reprocessing. It is possible to postpone it some days," Gholamreza Aghazadeh, the head of Iran's Atomic Organisation, told the Tehran state-run television in a live roundtable discussion on Thursday.
British Prime Minister Tony Blair said on Thursday he would support referring Iran to the UN Security Council if it breaches its nuclear obligations, sending a strong warning against any resumption of the reprocessing programme.
Conversion to resume
Iran announced on Sunday it would resume some conversion activities within days - Aghazadeh had said earlier it could come as soon as Thursday, expressing impatience with dead-locked negotiations with the Europeans over its nuclear programme.
The announcement sparked a European warning of negative consequences for Iran.
"There is no more room for withdrawal by Iran ... We cannot leave our 700 experts idle in the Isfahan conversion facility"
Head of Iran's Atomic Organisation
Thurdsday's comments by Aghazadeh, who is also the vice-president, suggested the pressure was having some effect - though he insisted Iran would not back down completely from its intention to resume some activities.
"A number of messages from different countries and Europeans are received by the country's officials, and some of them were discussed," he said.
But, he added: "There is no more room for withdrawal by Iran ... We cannot leave our 700 experts idle in the Isfahan conversion facility. If we give in (on the nuclear issue), then we have to withdraw in all disputed areas too because they (the West) oppose the Islamic Republic."
The government has faced pressure to resume suspended activities from some Iranian factions who accuse it of giving in to the Europeans, and the nuclear programme is a centre of national pride for many Iranians.
The United States accuses Iran of seeking to develop nuclear weapons, though Tehran insists its nuclear programme aims only to produce energy.
Iran says its nuclear programme
is only to produce energy
Iran suspended most nuclear activities amid the negotiations with Europe to avoid being referred to the Security Council, where it could face sanctions.
Reprocessing involves converting raw uranium into gas, which - in the next step - can then be enriched by being pumped into centrifuges. Enriched uranium can then be used to produce either fuel for a nuclear power reactor or to make nuclear weapons.