Iran's foreign ministry said Wednesday it would not suspend its sensitive high-level enrichment in return for radioisotopes, as offered in a letter by three world powers to the U.N. atomic watchdog.

"It is not at all rational to say that Iran should not produce [isotopes and uranium] and stop its [enrichment] plant and that they will provide its needed medicine," ministry spokesman Ramin Mehmanparast told the country's ISNA news agency.

"We will not examine offers which lead to the shutting down of the Tehran reactor."

Iran started enriching uranium to 20 percent on Feb. 9 to fuel a research reactor that makes medical radioisotopes, amid international concern over its atomic ambitions.