Catherine Ashton, foreign policy chief for the EU, said it would be "extremely difficult" to conclude an accord before the six-month deadline is reached, but it was more important to create an acceptable agreement.
"We have no guarantees in this and we will take the time that is necessary to get this to be the right agreement," Ashton told the Wall Street Journal during an EU conference in Munich, Germany.
Iran and six Western powers agreed in November to a six-month deal that would see Iran scaling back its nuclear program in exchange for an easing of some economic sanctions.
The pact went into effect Jan. 20 and can be extended if all parties agree. The Journal said many Western diplomats have expressed skepticism privately that a deal can be reached in six months due to the complexity of the negotiating issues.