US President George W Bush has said there is a "real and urgent" need for a missile defence system in Europe.
"Russia is not our enemy," said Mr Bush
Mr Bush said the missile threat was from the Middle East, not Russia, which strongly opposes sites for the shield in Poland and the Czech Republic.
He warned that Iran could have a ballistic missile capable of reaching Europe or the US by 2015.
Earlier, US Defence Secretary Robert Gates said the shield could be delayed while Russian concerns were tackled.
In a speech at the National Defence University in Washington, Mr Bush said: "The need for missile defence in Europe is real and I believe it's urgent."
He said the planned system was not designed to tackle missiles from Russia as it would be easily overwhelmed by Moscow's arsenal.
"The Cold War is over. Russia is not our enemy," he said.
Mr Bush said the US had invited Russia to "join us against an emerging threat that affects us all... we ought to respond to this threat together".
The president said if "rogue states" had less confidence their missiles would strike, they would be "less likely to engage in acts of aggression in the first place".
Mr Bush also attacked the US Congress for reducing funding to missile shield systems.
Earlier, Mr Gates had said activation of the European shield could be delayed until there was "definitive proof" of a missile threat from states such as Iran.
He said after meeting Czech Prime Minister Mirek Topolanek in Prague: "We would consider tying together activation of the sites in Poland and the Czech Republic with definitive proof of the threat - in other words, Iranian missile testing and so on."
The missile shield system would see a radar site set up in the Czech Republic and a missile interceptor base in Poland.
Russia has vehemently opposed bases on the territories of its former Warsaw Pact allies.
Mr Gates and US Secretary of State Condoleezza Rice received a frosty reception when they tried to sell the plan in Moscow this month.
Russian Foreign Minister Sergei Lavrov said Moscow saw the shield as a "potential threat" to its security and wanted to "neutralise" it.
Russian President Vladimir Putin widened the debate by also threatening to abandon a key nuclear missile treaty.
He said it would be difficult to remain part of the Intermediate Range Nuclear Forces (INF) treaty unless it was expanded to include more countries than just the US and Russia.
US MISSILE DEFENCE: LONG RANGE THREAT PROTECTION
US wants to build defence system against possible missile attacks
Part of defences would be in Eastern Europe - which Russia opposes
Russia suggests US should use its Gabala, Azerbaijan base instead