The leaders of Germany and France have rounded angrily on the US for the first time over spying claims, signalling that ambitious EU-US trade talks scheduled to open next week could become an early casualty of the burgeoning transatlantic espionage dispute.

The German chancellor, Angela Merkel, and French president, François Hollande, described the disclosures of massive US spying and snooping in Europe as unacceptable, with the Germans suggesting there had to be mutual trust if the trade talks were to go ahead in Washington on Monday.

Merkel delivered her severest warning yet on the National Security Agency debacle. "We are no longer in the cold war," her spokesman, Steffen Seibert, said. "If it is confirmed that diplomatic representations of the European Union and individual European countries have been spied upon, we will clearly say that bugging friends is unacceptable."

He said Berlin was keen on the trade talks with Washington, but qualified that support. "Mutual trust is necessary in order to come to an agreement."

After the Guardian's disclosure that US agencies were secretly bugging the French embassy in Washington and France's office at the UN in New York, Hollande called for an immediate halt to the alleged spying.

"We cannot accept this kind of behaviour between partners and allies," Hollande said. "We ask that this stop immediately."

The Europeans received their first opportunity to demand answers from the top level of the Obama administration about the massive scale of US spying on its EU allies when Lady Ashton and John Kerry met in Brunei. The EU and US foreign policy chiefs met on the fringes of a meeting of EU, US and south-east Asian governments, giving the British peer a chance to air EU grievances over the disclosures in the Guardian. On Sunday she demanded prompt clarification from the Americans over the veracity of the media reports.

Kerry, the US secretary of state, delivered a low-key response to the growing European clamour for answers, saying the NSA activities were not unusual. "Every country in the world that is engaged in international affairs of national security undertakes lots of activities to protect its national security and all kinds of information contributes to that," he said. "All I know is that is not unusual for lots of nations."

A sense of naked outrage gathered momentum across Europe at the reports that US agencies were bugging and tapping EU offices in Washington and New York, as well as the embassies of several EU member states. The push for clear answers from the Americans threatened to derail long-awaited talks on a transatlantic free trade pact between the US and the EU to create the world's biggest free-trade area.

"Washington is shooting itself in the foot," said Germany's conservative Frankfurter Allgemeine Zeitung newspaper. "Declaring the EU offices to be a legitimate attack target is more than the unfriendly act of a machine that knows no bounds and may be out of the control of politics and the courts."

François Hollande, who warned France would not tolerate spying. Photograph: LCHAM/Sipa/Rex Features

Martin Schulz, the president of the European parliament, likened the NSA to the Soviet-era KGB and indirectly suggested a delay in the talks.

Greens in the European parliament, as well as in France and Germany, called for the conference to be postponed pending an investigation of the allegations. They also called for the freezing of other data-sharing deals between the EU and the US, on air transport passengers and banking transactions, for example, and called for the NSA whistleblower, Edward Snowden, to be granted political asylum in Europe. French Greens asked Hollande to grant Snowden asylum in France.

Schulz said: "I feel treated as a European and a representative of a European institution like the representative of the enemy. Is this the basis for a constructive relationship on the basis of mutual trust? I think no.

"We are just starting negotiations on a free-trade agreement between the European Union and the United States. This is not the basis to build mutual trust, this is a contribution to build mutual mistrust and, therefore, I understand all those in Europe who think that we should first of all ask the Americans, secondly look to or listen to the justification and then take the necessary consequences.

"On the other hand, with this affair or not, the United States of America and Europeans remain … strategic allies. Therefore it is shocking that the United States take measures against their most important and nearest allies, comparable to measures taken in the past by the KGB, by the secret service of the Soviet Union."

While the anger is broad and growing across Europe, it is particularly intense in Germany which, according to Snowden's revelations, is by far the main target within the EU of the NSA's Prism programme sweeping up metadata en masse, capturing and storing it.

Given the high sensitivity of data privacy issues in Germany, the scandal could test Merkel and force her on to the offensive against the Americans as she seeks to win a third term in general elections 11 weeks away.

The opposition Social Democrats in Berlin demanded action from Merkel, but left her scope to cut a deal that would allow some snooping and data exchanges. Frank-Walter Steinmeier, the Social Democrats leader in the German parliament, said the chancellor had to insist "the mania for data collection be palpably limited".

But he also called for a durable agreement between the Germans, the Americans and the British, clarifying what was needed on security grounds and what damaged civil liberties.

In addition to the furore over the NSA and the Americans, the Germans are incensed at the British because of GCHQ's Tempora programme which is gathering electronic information from across Europe.

John Kerry, US secretary of state, in Brunei. He downplayed reports of NSA spying on its allies. Photograph: Made Nagi/EPA

The Germans are to be given their first proper opportunity to be briefed by the British on Monday afternoon, according to Der Spiegel. London has called a video conference with the Germans at the British embassy in Berlin. The Germans are sending intelligence officers, diplomats, and officials from the interior and justice ministries to take part, the news magazine reported.

Germany's federal prosecutor's office has also opened inquiries into the NSA debacle, with a view to establishing whether German laws have been breached.

"The prosecutor's office is carefully assessing the media reports with reference to its legal mandate," a spokeswoman said. It would examine the available information on the Prism, Tempora, and Boundless Informant programmes and seek to establish whether the NSA's interception of telephone and internet communication was violating German laws, she added.

"The Americans justify everything with combatting terrorism," said the Luxembourg foreign minister, Jean Asselborn, who on Sunday described the latest allegations as disgusting. "The EU and its diplomats are not terrorists."

Elmar Brok, the veteran MEP who chairs the European parliament's foreign affairs committee and is from Merkel's Christian Democratic party, said the opening of the trade talks next week had been jeopardised. "How can you negotiate when you have got to fear that your own negotiating position has been intercepted in advance?" he asked.

Austria's Hannes Swoboda, head of the social democrats in the European parliament, said: "We demand full disclosure on the alleged bugging and wire-tapping of EU representatives by the US authorities, including the potential involvement of EU member states' intelligence services. The EU and US have to see eye to eye in this world and share relevant information. Spying is certainly not the right way to reinforce co-operation.

"If partners become targets, we may want to review our positions. This includes negotiations on a TTIP (Transatlantic Trade and Investment Partnership). It is meant to be the way by which Europe and the USA will strengthen their ties, but, should our citizens, enterprises or economies not be protected or even spied upon, we may want to think it over."

Harlem Désir, the head of the French Socialist party, told France Info radio that if the latest revelations were true, "it would be an unacceptable scandal". He said the spying allegations were of such a nature that they could call into question the EU-US negotiations. He added: "The fight against terrorism cannot explain everything."

France's justice minister, Christiane Taubira, said, if confirmed, the US behaviour was of unspeakable hostility.