US President George W Bush has refused to rule out military action against Iran over its nuclear programme.
Bush urged Europe to keep the pressure on Iran
However, Mr Bush said diplomacy was his "first choice" to resolve the stand-off, stressing that Tehran "should not have a nuclear weapon".
His comments on Belgian TV came just hours after Russian President Vladimir Putin said Iran had convinced him it was not trying to build nuclear arms.
Moscow is currently helping Iran build a nuclear reactor.
It is a project which has been heavily criticised by the US, which accuses Tehran of seeking nuclear weapons .
Tehran denies this, saying its nuclear development programme is purely for peaceful, energy-generating purposes.
Earlier this week, Iran vowed to back Syria against "challenges and threats" as both countries continued to face strong US pressure.
"First of all you never want a president to say never, but military action is certainly not, is never the president's first choice," Mr Bush told Belgium's TV channel VRT, when asked if he could rule out a military strike against Iran.
The Bushehr reactor is being built with Russian assistance
But he stressed that "diplomacy is always the president's or at least always my first choice".
The interview was recorded in Washington earlier and aired before President Bush arrival in Brussels for summits with Nato and the EU on Sunday.
President Bush said that the US and the EU had "a common goal and that is that Iran should not have a nuclear weapon".
"I think if we continue to speak with one voice and not let them split us up and keep the pressure on, we can achieve the objective."
EU negotiators have recently offered to replace Iran's heavy-water nuclear reactor - which can be used to make weapons-grade nuclear material - with a light-water reactor.
Low grades of uranium are used for nuclear reactor fuel, but higher grades can be used in atomic bombs.
Iran suspended enrichment temporarily in November as part of a dialogue process with the European Union.
President Putin pledged on Friday to continue co-operation with Iran in all fields, adding that he had accepted an invitation to Iran.
His comments came at a meeting in Moscow with chief Iranian nuclear negotiator Hassan Rowhani.
Under an agreement announced on Thursday and due to be signed this month, Moscow will supply Tehran with the nuclear fuel it needs.
The spent fuel will be returned to Russia. This was the last issue delaying the start of operations at the Russian-built reactor at Bushehr, in southern Iran.
The US believes that the Bushehr reactor - when completed - could enable the Iranians to extract weapons grade plutonium.