Brussels ? Dozens of Iranian banks were blocked from doing business with much of the world as the West tightens the financial screws on a country it wants to prevent from developing nuclear weapons.

The Belgium-based company that facilitates most international bank transfers on Thursday took the unprecedented step of blocking 30 Iranian banks from using its service. The move is likely to hurt Iran’s all-important oil industry and make it difficult for citizens to receive money from relatives living abroad.

The move by the Society for Worldwide Interbank Financial Telecommunication, or SWIFT, is part of a broader effort by Western nations to isolate Iran financially and force it to demonstrate that it is not trying to develop nuclear weapons. Iran says that its nuclear program is for peaceful purposes only, but officials in many other countries believe otherwise.

SWIFT said it was forced by recent European Union sanctions to discontinue service to the Iranian banks beginning Saturday. SWIFT is a secure private network used by nearly every bank around the world to send payment messages that lead to the transfer of money across international borders.

The chief executive of SWIFT, Lazaro Campos, described the move as “extraordinary and unprecedented.”

“It is a direct result of international and multilateral action to intensify financial sanctions against Iran,” he said.

There was no immediate reaction from the Iranian government or the banks involved. Not all Iranian banks are subject to EU sanctions, and oil experts say there will be ways for Iran to sell oil without using SWIFT.