Washington says Iran has no need to continue its own nuclear programme now that Russia has started delivering fuel to the Bushehr power plant.
Bushehr is only the first reactor in Iran's nuclear programme
But Tehran says it will not stop the uranium enrichment process despite the threat of further UN sanctions.
It is the first time Iran has received a fuel delivery from Russia, which is building the Bushehr plant.
Tehran insists it is pursuing peaceful power generation only, despite fears it could try to build a bomb.
The arrival of the first Russian shipment came days after Moscow and Tehran agreed on a schedule to finish building the Bushehr plant after years of delays.
While the UN continues to demand that Iran halt uranium enrichment, it has approved the Russian deliveries.
'Still a threat'
Confirming that Moscow had notified the US about the shipment, President George W Bush said the Russian shipment proved the Iranians did "not need to learn how to enrich" uranium for themselves.
BUSHEHR NUCLEAR PLANT
Begun in 1974 with German assistance
Work halts after Iran's 1979 Islamic revolution
Resumed in 1992 with Russian help
13 Dec: Russia and Iran agree to finish plant after numerous delays
Two pressurised water reactors
One believed near completion, could begin operating in eight months
He said that Iran remained a "threat to peace" if its own enrichment, a crucial step towards constructing nuclear weapons, was not stopped.
"They're heading down a path of isolation right now and economic sanctions," he added.
"We passed two resolutions out of the UN and [US Secretary of State] Condi Rice is working on a third."
After a recent US intelligence report which found that Iran had stopped an alleged nuclear weapons programme in 2003, Mr Bush said Tehran could not be trusted.
"If somebody had a weapons programme, what's to say they couldn't start it up tomorrow?", he asked earlier this month.
"Since they tried to hide their programme before, how would we know?"
Atomstroiexport, the Russian company building the Bushehr plant, said the delivery of the enriched uranium fuel began on Sunday.
The final delivery, it said, would be made in February. Russian officials said earlier that the plant could be operational within six months of fuel being delivered.
Gholam Reza Aghazadeh, head of Iran's atomic energy agency, said the plant would begin operating some time in the new year.
He stressed that nuclear enrichment work would not stop as a result of the Russian deliveries, saying fuel was needed for a reactor under construction in Darkhovin, south-west Iran.
Enriched uranium is used as fuel in nuclear power stations. When it is more highly enriched, it can be used to make nuclear weapons.