Prosecutors said Wednesday they were probing more than 100 incidents in which construction foam was used to block exhaust pipes, in a campaign that infuriated the owners and sparked a backlash against environmental activists.

News site Der Spiegel said security services now suspect Moscow of orchestrating the campaign, in which stickers were placed on car windows with the message "Be greener!" and the smiling face of Greens Vice Chancellor Robert Habeck.

According to Der Spiegel, one suspect told police that a Russian man had commissioned the vandalism, asked for photos of the damage as proof and promised the perpetrators a 100 euro ($104) bonus per car.

No German prosecutors or officials have confirmed that Russia is thought to be behind the campaign, but Foreign Minister Annalena Baerbock suggested it fits Moscow's playbook at a time of soaring east-west tensions over the Ukraine war.

Baerbock, also from the Greens, said the latest reports of sabotage showed that "the Kremlin has been trying for a long time to destabilise European democracies using different methods".

"Whoever stands against Russia's illegal war of aggression comes into the crosshairs of the Kremlin and its henchmen," said a statement by Baerbock, herself a frequent target of online abuse and disinformation.

Prosecutors in the southern city of Ulm said four people aged between 17 and 29, from countries including Romania, Serbia and Bosnia are suspected of involvement in more than 100 incidents in which car exhaust pipes were blocked with the hardening foam.

They were being investigated for "damage to property" in the sabotage attacks that caused thousands of euros worth of damage.

- Deepening divisions -

According to Der Spiegel, more than 270 similar acts of vandalism have been reported in total across Germany, many using stickers with Habeck's face.

The magazine said security services consider it "a targeted campaign with the intention of stirring up hatred against the Greens and their candidate for chancellor, Habeck, in the federal election campaign".

The sabotage came at a time activists alarmed by global warming have also glued themselves to roads, causing traffic jams, and carried out other actions that have often sparked heated debate and fury from motorists and others.

In December, prosecutors in the state of Brandenburg said they were also investigating incidents in which 43 cars were damaged with construction foam and had "stickers with political content" left on their windows.

An interior ministry spokesman told a press conference on Wednesday that so far there was "no firm proof of Russian involvement" in the vandalism.

But he recalled that "we have already seen a whole range of actions by Russian intelligence agencies or Russian-led actors... including sabotage".

Germany's intelligence services have been sounding the alarm for months about Russian attempts to meddle in German politics, particularly ahead of February 23 elections.

They have warned that Moscow and its allies may try to deepen existing rifts and divisions in German society.

The Greens have faced particular attacks from the far right over their climate policies and other issues, including on migration and gender policies.

The party has also been among the staunchest backers of military aid to Ukraine to help it repel Russia's invasion.

Other countries have also accused Russian-led actors of trying to inflame domestic divisions.

French authorities said a year ago they believed that Russia's security service FSB was behind a campaign around the start of the Israel-Hamas war in late 2023 in which Star of David graffiti were daubed on buildings in and around Paris.