The United States has pressured Russia over its assistance to Iran's civilian nuclear programme, which some Western leaders fear could be adapted to produce nuclear weapons.
Russia is building Iran's first nuclear reactor at Bushehr despite protests from Israel and the US.
"What Russia has agreed to do is to send highly enriched uranium to a power plant, then collect that uranium," Bush said in a televised prime time news conference on Thursday.
"I appreciate that gesture, what they recognised and what America recognises and what Great Britain and France and Germany recognises is that we cannot trust the Iranians when it comes to enriching uranium.
"What Russia has said is: 'We will provide you the uranium, we will enrich it for you and provide it to you and then we will collect it.'"
Iran has repeatedly said it is not seeking weapons of mass destruction and that its nuclear activities are for peaceful purposes.
Talks on hold
Planned Iran-EU nuclear talks on Friday in London are expected to show both sides feel the negotiations are on hold until after Iranian presidential elections in June, diplomats said.
"It is still not the end game. Look to June," a European diplomat close to the talks said.
Another diplomat said the talks, expected to be in the evening, would be at the level of political directors of foreign ministries but held secretly and away from the press.
In Tehran on Thursday a key Iranian negotiator said the Islamic republic had become pessimistic since the European Union began dragging its feet.
The EU wants Iran to give up its
uranium enrichment programme
Hossein Mussavian said that since the start of the process in December, and especially since the Paris meeting on 23 March, the Europeans had not undertaken any serious steps to bring the issue to a close.
Iran is waiting for an answer from EU negotiators Britain, Germany and France on a proposal that would allow Iran to enrich uranium, a process that makes fuel for nuclear reactors but in highly refined forms can be the explosive core of atom bombs.
The European Union would like Iran permanently to give up uranium enrichment.
In exchange, the three European governments are offering Iran a package of trade, security and technology incentives.