French Foreign Affairs Minister Laurent Fabius on Sunday called on Washighton to explain spying allegations on European Union (EU) offices in Washington, Brussels and at the United Nations.

"We are waiting the U.S. authorities to answer as soon as possible the legitimate questions raised by press report," Fabius said in a statement.

"If these allegations would be confirmed, they would be totally unacceptable," he added.

On Saturday, the German magazine Der Spiegel reported on its website that the U.S. National Security Agency had tapped EU offices and gained access to EU internal computer networks.

The magazine said its report was based on confidential documents, some of which it had been managed to consult via fugitive ex-CIA analyst Edward Snowden.

Washington has not responded to the questioning yet.

Representing almost the half of the world's total output, the U.S. and the European Union started in June talks to forge a free trade accord expected to spur growth and create jobs on both sides of the Atlantic.

However, the alleged U.S. spying case provoked politicians' outrage in the European bloc and in Paris where they called for immediate freeze of trade negotiation.

"If the spy allegations confirmed, at least we should suspend negotiations of the transatlantic free trade accord," Jean-Christophe Cambadelis, national secretary of the Socialist Party in charge of Europe and foreign affairs said in a communique.

For his part, Jean-Luc Melenchon, leader of the Left Front urged Europeans to immediately put talks on ice and asked France to offer political asylum to Edward Snowden who "helped to unmask this plot," he was quoted as saying by the channel news BFMTV.?