Steinmeier, a Social Democrat who served as Foreign Minister under Chancellor Angela Merkel before being elevated to the presidency, said Russia's invasion of Ukraine meant he and others had to reckon honestly with what they had got wrong.

"My adherence to Nord Stream 2 was clearly a mistake," he said. "We were sticking to a bridge in which Russia no longer believed and which other partners had warned us against."

Steinmeier was a prominent member of a wing of his Social Democratic Party, led by former Chancellor Gerhard Schroeder, that argued close economic ties to Russia were a way of anchoring it within a western-oriented global system.

The now-cancelled Nord Stream 2 pipeline, which critics said would have weakened Ukraine by cutting it out of the energy transit business, was a centrepiece of that strategy.

That has triggered a growing backlash, with critics on social media repeatedly tweeting past pictures of him affectionately embracing Russian Foreign Minister Sergei Lavrov, while Ukraine's ambassador Andrij Melnyk has been outspoken in his criticism.

When Steinmeier arranged a "solidarity concert" for Ukraine, Melnyk tweeted sarcastically that the only soloists appeared to be Russian. "An affront," he wrote. "Sorry, I'm not coming."

Germany's president is meant to be a unifying figure who stands above the cut and thrust of daily politics, one who enjoys the moral authority to exhort people to better behaviour.

"We failed to build a common European house," Steinmeier said. "I did not believe Vladimir Putin would embrace his country's complete economic, political and moral ruin for the sake of his imperial madness," he added.

"In this, I, like others, was mistaken."