"We call on Iran's leadership immediately to suspend its sensitive nuclear activities and abide fully by its international obligations," British Prime Minister David Cameron, French President Nicolas Sarkozy and German Chancellor Angela Merkel said in a joint statement.
"Our message is clear. We have no quarrel with the Iranian people. But the Iranian leadership has failed to restore international confidence in the exclusively peaceful nature of its nuclear programme," they said.
"We will not accept Iran acquiring a nuclear weapon. Iran has so far had no regard for its international obligations and is already exporting and threatening violence around its region."
The three leaders said "the door is open to Iran to engage in serious and meaningful negotiations about its nuclear programme.
"Until Iran comes to the table, we will be united behind strong measures to undermine the regime's ability to fund its nuclear programme, and to demonstrate the cost of a path that threatens the peace and security of us all."
The EU slapped an embargo on Iran's oil exports as part of the package of new sanctions Monday aimed at blocking funds for Tehran's suspect nuclear drive and pressing it to return to talks.
The oil ban, along with sanctions against Iran's central bank and other measures, come amid mounting concerns of confrontation after the UN atomic agency reported Tehran was inching ever closer to building a nuclear bomb.
The EU has already frozen the assets of 433 firms and 113 individuals, as well as restricting trade and investment in the oil and gas industries.
The bloc imported some 600,000 barrels of Iranian oil per day in the first 10 months of last year, making it a key market alongside India and China, which has refused to bow to pressure from Washington to dry up Iran's oil revenues.
"The embargo mustn't take place solely in the EU, in the West," said Germany Foreign Minister Guido Westerwelle. "For the message to the Iranian regime to be clear it needs more than a western voice, it needs an international voice."
Some 65 percent of Iranian exports go to Asia.
In Europe, Iranian oil accounted for 34.2 percent of Greece's total oil imports, 14.9 percent of Spain's and 12.4 percent of Italy's in the first nine months of last year, according to the latest EU statistics.
The bloc thus has been seeking new suppliers able to match the attractive conditions offered by Tehran. Contacts are under way with Saudi Arabia and hopes are high that Libya can soon increase its production.