Western governments say the higher-grade enrichment marks a notable step towards weapons-grade uranium, even though it is below the 90 percent level suitable for nuclear bombs.
Iran says its enhanced enrichment is to make fuel for a Tehran research reactor that produces isotopes for medical care.
Diplomats in Vienna, where the U.N. nuclear watchdog agency is based, said on Monday they had no knowledge of any incident at Fordow but were looking into the reports. One Western diplomat said he did not believe them to be correct.
The U.N. International Atomic Energy Agency, which regularly inspects Iranian nuclear sites including Fordow, had no immediate comment.
Iran has accused Israel and the United States of trying to sabotage its nuclear programme with cyber attacks and assassinations of its nuclear scientists. Washington has denied any role in the killings while Israel has declined to comment.
(Additional reporting by William Maclean and Marcus George in Dubai, Justyna Pawlak in Brussels, Fredrik Dahl in Vienna; Writing by Mark Heinrich; Editing by Robin Pomeroy)