Kerry met with British Foreign Minister William Hague as representatives from the five permanent members of the U.N. Security Council, Germany and Iran prepare to sit down in Kazakhstan to discuss nuclear issues.
Kerry said he shared Britain's concerns about the potential for a nuclear-armed Iran.
"As we've said again and again, an Iran with a nuclear weapon in that region, and given all that has happened, is simply unacceptable," Kerry said during a news conference.
Iran's relationship with Syria has sparked concern the Syrian civil war, Tehran has also been accused of interfering in conflicts in Yemen.
Iran recently met with the International Atomic Energy Agency to discuss a nuclear program that Tehran says is for peaceful purposes.
Kerry said the window of opportunity to find a diplomatic solution to the issue "simply cannot by definition remain open forever."
Hague said the British government was approaching the Iran talks in good faith.
"However Iran should not doubt our resolve to ensure that nuclear proliferation in the Middle East is prevented," he said.
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