"What right do people have to say the US and its allies can't defend themselves?"
Sharmad, Seattle, US
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"The need for missile defence in Europe is real and I believe it's urgent.
"If [Iran] chooses to do so, and the international community does not take steps to prevent it, it is possible Iran could have this capability. And we need to take it seriously, now.
"Today we have no way to defend Europe against the emerging Iranian threat. And so we must deploy a missile defence system there that can."
But Robert Gates said the US may delay activating part of the shield until there is clear evidence of a threat and called for Russia to co-operate in the project.
"We would consider tying together the 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," he said.
The US wants to place interceptor missiles in Poland and a radar station in the Czech Republic.
Washington says the system is needed to counter the threat of missile attacks from "rogue states", including Iran and North Korea but Russia argues that it is a threat to Moscow's security.
"We continue to encourage the Russians to partner with us in missile defence and continue our efforts to reassure them that these facilities are not aimed at Russia and could benefit Russia," Gates told reporters in the Czech capital Prague.
The US also offered to allow Russia to maintain a presence at the Czech radar station, to promote transparency.
However, Gates said nothing would be agreed without the approval of the Czech government.
Much of the disagreement between Washington and Moscow over the system has centred on the immediacy of the threat from Iranian missiles on Europe and the US.
Sergey Lavrov, the Russian foreign minister, said on a visit to Tokyo that the US initiative was based on an erroneous assessment of the threat posed by Iran.
"North Korea poses a fundamental threat, but Iran does not," he was quoted as telling Masahiko Komura, his Japanese counterpart.
Agreement on the system could be delayed after last weekend's election in Poland brought to power a party that has promised tougher negotiations over the plan.
The Czech Republic also said on Tuesday that it may be six months before a deal can be finalised and approved by parliament.