Mohammad Saeedi, the deputy head of the Atomic Energy Organisation of Iran, said on Tuesday that officials of the UN nuclear watchdog body had authorised the seals' removal on Monday night.
"Today, with the powers delegated to IAEA inspectors in Iran, some of the seals that are in the field of only research were removed, and research facilities resume their work," Saeedi said.
Inspectors from the International Atomic Energy Agency (IAEA) arrived in Tehran on Saturday to remove the seals put on the nuclear research sites more than two years ago.
The IAEA confirmed that Iran has broken UN seals at the Natanz uranium enrichment plant, a move likely to add to tensions between Tehran and the West.
"I can confirm that the Iranians have begun removing (UN) seals at Natanz in the presence of IAEA inspectors," IAEA spokeswoman Melissa Fleming said.
The move is likely to increase pressure on Iran from Western nations which have called for it to cease nuclear activities until an agreement has been reached on the scope of its nuclear programme.
Many Western nations, led by the US, say Tehran is using the Natanz facility and other plants as a cover for developing atomic weapons.
The latest Iranian move will add
to tensions with the West
Russia's parliament also expressed concern over the latest development. Constantine Kovatchev, chairman of the International Relations Committee, told Aljazeera that Moscow could reconsider its nuclear cooperation with Tehran.
Iran, however, insists its nuclear programme is meant only for civilian purposes.
Saeedi stressed that Iran was not resuming the production of nuclear fuel, a process that would involve uranium enrichment.
"What we resume is merely in the field of research, not more than that," he said. "We make a difference between research on nuclear fuel technology and production of nuclear fuel. Production of nuclear fuel remains suspended."
"The latest information that Iran has announced its intention in the near future to restart work connected with the enrichment of uranium provokes concern"
Russian Foreign Minister
Tehran's drastic measure to resume nuclear fuel research drew instant reactions from across the world.
Russian urged Iran to stick to its nuclear commitments and said it was concerned about Tehran's latest moves, Russian news agencies quoted Sergei Lavrov, the foreign minister, as saying.
"The latest information that Iran has announced its intention in the near future to restart work connected with the enrichment of uranium provokes concern," Lavrov said.
Britain condemned Iran and said the move put further negotiations with Britain, France and Germany at risk.
"This is a very negative development and seriously jeopardises the negotiating process," said a spokesman for Britain's foreign office.
"Britain, France, Germany and (EU foreign policy chief Javier) Solana's office are in close touch and we expect ministers to discuss next steps soon."
France also took a dim view and said both Iran and North Korea would be making a serious error if they refused to engage with the West over the nuclear programme.
"These countries would be committing a serious mistake if they
did not take the hand that we are holding out to them," Jacques Chirac, the French president, said.