German Chancellor Angela Merkel said Saturday that European politicians "must stand up to" far-right politicians "for sale", after a hidden-camera sting brought down nationalist Austrian Vice-Chancellor Heinz-Christian Strache over allegations he was open to corruption.

"We are confronted with currents... who want to destroy the Europe of our values, and we must stand up to that decisively," Merkel told a press conference in the Croatian capital Zagreb before heading to a gathering of the country's ruling conservative HDZ party ahead of European Parliament elections.

While far-right parties rejected values like protecting minorities and basic human rights, Merkel said, "politicians being for sale plays a role, and we must act decisively against all of that".

Germany's Der Spiegel and Sueddeutsche Zeitung late Friday published video recordings, allegedly showing Freedom Party (FPOe) leader Strache promising public contracts in return for campaign help from a fake Russian backer.

Merkel's intervention followed a string of appeals from leading German politicians for voters not to follow the neighbouring country's example by putting the far right in positions of power.

"People like this mustn't be allowed to take any responsibility in Europe," the leader of Merkel's centre-right CDU party Annegret Kramp-Karrenbauer said.

Austria's conservative Chancellor Sebastian Kurz had been "irresponsible" to invite the FPOe into his government, foreign minister Heiko Maas of the centre-left Social Democratic Party (SPD) said, calling them "enemies of freedom".

Other parties preemptively warned the CDU off potential alliances with German far-right outfit Alternative for Germany (AfD).

Votes later this year in Germany's former communist East look set to hand the party strong results that could make it hard to build state-level coalitions without them.

Centre-right parties across Europe "should end their alliances with right-wingers and distance themselves clearly from the enemies of democracy," Greens lead candidate for the EP Sven Giegold told the Funke newspaper group.

Although in the past they have presented themselves as bosom friends, there was no reaction to Strache's resignation from leading AfD figures by late afternoon Saturday.

Joint AfD leader Joerg Meuthen was in Milan for a gathering of nationalist parties organised by Italian deputy prime minister Matteo Salvini.

The FPOe's lead candidate for the European Parliament pulled out of the meeting after the scandal broke.

Apart from Merkel and Prime Minister Andrej Plenkovic, the HDZ event in Zagreb was attended by German MEP Manfred Weber, the candidate of the European People's Party (EPP), the main centre-right bloc in the assembly.

Weber is the apparent frontrunner to replace Jean-Claude Juncker as president of the European Commission.

"I'll fight against the nationalists, I'll fight against the populists, they will not destroy our today's EU," Weber told several thousands of HDZ supporters at a Zagreb sports hall.

He pledged not to work in "any moment and on any single issue with all the right populists in the next European Parliament.

"They will not be part of majority of Manfred Weber. I'm clear on this," he said.

HDZ will win five or six out of 12 Croatian seats in the European Parliament, surveys predict.