Six world powers, including Britain, are due to meet Iranian officials for another round of negotiations over the nuclear issue in Baghdad on May 23.
Hague said the EU wanted to see evidence that Tehran was now taking specific steps to reassure the world that its nuclear program does not have a military dimension as many countries suspect.
“Now we wait to see some concrete steps and proposals from Iran,” Hague told reporters before a meeting of EU foreign ministers in Brussels.
“Without that, of course we have sanctions we have imposed. They will not only be enforced but, over time, intensified.”
Negotiations over Iran’s nuclear program resumed last month after a gap of more than a year, offering a chance to ease tensions and avert the threat of a new war in the Middle East. Iran says its nuclear program is purely for peaceful purposes.
While all sides described the atmosphere in April as positive, Tehran has so far offered few concrete concessions and continues to argue it has the right to a nuclear program for domestic energy, research and other purposes.
It is facing a raft of intensified international sanctions, including an EU ban on its crude oil exports, which is due to take full effect on July 1.
Tehran has said sanctions should be eased if the international community wants progress in the talks.
But the West in particular has made it clear it will only relax the sanctions if it sees Iran becoming more transparent and cooperative.
The ultimate aim is to pressure Tehran into halting work that it is feared may be intended to give the Islamic republic a nuclear weapons capability.
