Tehran says it received a request from the head of the UN's nuclear watchdog agency for the delay, but the International Atomic Energy Agency (IAEA) denied Mohammed ElBaradei had made such a request.
"We have sent a letter to Iran indicating that it would take at least a week to get our surveillance equipment and other required measures in place," IAEA spokeswoman Melissa Fleming said.
Earlier, Ali Agha Mohammadi, spokesman for Iran's Supreme National Security Council, said Iranian technicians would break the seals and restart nuclear processing on Monday.
He said the combination of restraint and resolve towards restarting uranium processing showed the government's intent not to squander Iran's right to nuclear power, while preserving close ties to Europe.
"Our people were worried that the government may have done a deal with the Europeans and given up the rights of the nation," Mohammadi said. "We will do the rest of the work in coordination with the Europeans."
On state-run TV late Monday, Mohammadi said authorities would delay opening Iran's Isfahan Nuclear Conversion Facility for a week if it thought European negotiators would offer a proposal that left its rights to nuclear technology intact.
Earlier in the day, ElBaradei warned Iran "not to take any action that might prejudice the process at this critical stage".
EU negotiators have said they are mere days from delivering a package of incentives addressing security and political, economic and nuclear issues.
"I also call on Iran not to take any unilateral action that could undermine the agency inspection process at a time when the agency is making steady progress in resolving outstanding issues," ElBaradei said.
Iranian officials had signalled an intensifying impatience with the slow pace of negotiations with Europe, and an incoming conservative administration in Tehran has showed signs of wanting to harden the country's stance.
Ali Agha Mohammadi says some
IAEA staff have already arrived
Mohammadi said authorities still plan to remove the UN's seals on the machinery, in the Isfahan plant, opening the way for the long-stalled conversion of uranium ore concentrate, known as yellowcake, into uranium gas, the feedstock for enrichment.
Iranian officials made clear they were still holding back from restarting most of their suspended programme. The country has no plans to reopen the plant in Natanz, where it could begin actual enrichment by injecting uranium gas into centrifuges used to enrich uranium.
In its letter to the IAEA, Tehran said the EU proposal sought to restrict the country's peaceful development of nuclear power while falling short on economic, technological and nuclear cooperation. It said the EU's "security guarantee" that Iran would not be invaded was also lacking.
Iran's announcement brought sharp responses from European officials who called on the Iranians to respect the terms of the Paris Agreement that meant the nuclear programme stayed frozen until negotiations were finished.
Mohammadi said some IAEA inspectors have already arrived in Isfahan. Others are expected to arrive to install more monitoring cameras to record the resumption of work.
"One more time, we ask the Iranians to respect our agreement, and not look for pretexts to make unilateral decisions," French Foreign Minister Philippe Douste-Blazy
told France-Info radio late Sunday.
US officials claim the Iranian nuclear programme is designed to produce weapons. Iran insists electricity is its sole aim. Iran maintains its suspension of uranium enrichment in November was voluntary, giving it the right to resume the activities.
Iran's moves could lead to the country being hauled before the United Nations Security Council to face sanctions, as previously called for by the United States. European diplomats said on Sunday that if Isfahan were restarted, an emergency IAEA board meeting would set a deadline for the Iranians to stop enrichment activities.
If such a deadline were not met, a Security Council referral was a likely next step, the officials said. Iran's actions could also trigger a short-term economic penalty.
Germany, which along with Britain and France have been leading US-backed EU negotiations, said on Monday European negotiators still plan to submit their proposal for Iran's atomic programme in a few days.
The proposal, which was delayed a week until 7 August, includes nuclear fuel, technology, other aid and "security guarantees" that Iran will not be invaded if it permanently halts uranium enrichment and related activities, European and Iranian officials confirmed.