"The countries that feel threatened ... should prepare for defense, and even counterattack"
Adolfo Talpalar, Stockholm, Sweden
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Bush's language appears to be a reaction to remarks by Vladimir Putin, the Russian president, who said he has not seen any evidence that Iran is seeking nuclear weapons.
Hours earlier, Putin made a proposal to end the nuclear crisis in talks with Mahmoud Ahmadinejad, the Iranian president, in Tehran.
Iranian officials said Putin proposed to break the deadlock over Iran's nuclear programme but gave no further details.
Commenting on the meeting, Bush said he wanted to get the Russian leader's "read-out from the meeting".
"The thing I'm interested in is whether or not he continues to harbour the same concerns that I do," Bush said.
Hillary Mann, a former White House official focusing on the Middle East, told Al Jazeera that Bush's comments signalled that the administration had stepped up its rhetoric to another level.
|Putin, right, made undisclosed proposals|
to end Iran's nuclear crisis [Reuters]
She said the new definition of threat - nuclear knowledge - was in line with the Israeli position.
Bush said he wanted Putin to explain his statement last week after meeting Nicolas Sarkozy, the French president, that Russia had no information about Iran's bomb-making intentions.
"I look forward to having him clarify those [comments]," Bush said.
"Because when I visited with him, he [said he] understands that it's in the world's interests to make sure that Iran does not have the capacity to make a nuclear weapon."
The sticking point is Iran's refusal to suspend uranium enrichment, which the West maintains could be diverted towards making a nuclear bomb.
Iran insists it wants to generate electricity, but the United Nations has so far imposed two sets of sanctions on the country over its nuclear programme.