"It must be stated that it goes against the decisions of both the IAEA and the UN Security Council," Gatilov said at a press conference in Moscow on Tuesday.
Uranium enriched at the 20-percent level can easily be made into fissile warhead material.
"It is important that what's happening at that site [in the Fordo Fuel Enrichment Plant] is being done under the strict supervision of the IAEA, but, in any case, it does not bring any benefit," Gatilov said.
"Unfortunately, Iran's behavior at times creates difficulties for beginning political negotiations," he said.
Tensions have been rising in the Persian Gulf after the U.S. signed into law sanctions against Iranian oil exports late last year, in an attempt to put pressure on Iran to abandon its nuclear program. European countries are expected to impose their own sanctions against Iranian oil next week, Reuters reported.
Deputy Prime Minister Dmitry Rogozin said Monday that Russia would consider any military intervention aimed at dismantling Iran's nuclear program a threat to its own security.