The joint decision comes just two days after the US National Intelligence Estimate said that international pressure may have led Iran to stop its nuclear weapons programme in 2003.
While the US pursued efforts to impose new UN sanctions, the EU tried to persuade Iran to end uranium enrichment in exchange for political and economic concessions.
"The reason we have to sanction on one hand and to deal on the other hand remains entirely valid," De Gucht said.
"The whole process of enrichment only makes sense when it's part of a process of producing a nuclear weapon, if not the whole process does not make sense."
He said Iran could decide to produce an atomic weapon as soon as "they have the fissile fuel".
On the defensive
On Tuesday George Bush, the US president, had said he was surprised when he was told of the findings on the mothballing of Iran’s nuclear weapons programme by his intelligence chief.
"I was made aware of the NIE last week. In August, I think it was Mike McConnell came in and said, 'we have some new information'," Bush said.
"He didn't tell me what the information was; he did tell me it was going to take a while to analyse."
In Washington, the White House was on the defensive about how Bush has dealt the NIE report.
Dana Perino, a White House spokesperson, said: "Look, I can see where you could say that the president could have been more precise in that language ... but the president was being truthful."