At least two German companies are reportedly involved in Russia’s reconstruction of Mariupol, a project the Kremlin is using to solidify its claim to the illegally annexed territory.

Knauf, a Bavaria cement maker, and WKB Systems, a concrete producer from the Rhine region, have both delivered material to construction firms working in the city, according to German broadcaster ARD.

The broadcaster found photographic evidence that cement sacks bearing Knauf’s logo were stacked up at building sites in the city, which Russia occupied after a long and bloody siege in 2022.

A construction firm working on a contract from the Russian defence ministry also advertised the fact that it was using Knauf cement in its PR material.

Despite pressure put on German companies to cut business ties in the wake of the 2022 invasion, Knauf has maintained a factory that employs some 4,000 people in Russia.

Nikolaus Knauf, the head of the family firm, was an honorary consul for Russia for several years and has been pictured with Vladimir Putin.

The company said in response that it produces cement in Russia “exclusively for the Russian market”.

It added that it condemns Russia’s war against Ukraine and “complies with all EU sanctions against Russia”.

Roderich Kiesewetter, foreign affairs spokesman for the centre-Right Christian Democrats, the main opposition party in the German parliament, said: “A company like Knauf that continues to do business in Russia is part of the Russia war economy and profits from Russia’s war.

“Knauf is in the truest sense of the word cementing Russia’s power in occupied territories such as Mariupol.”

The broadcaster has also published photographs of concrete blocks wrapped in plastic bearing the logo of WKB Systems, a German firm that belongs to Viktor Budarin, a Russian oligarch.

Customs data shows that the company supplied the components to construct entire concrete production facilities to Russia over a period of several years.

According to the campaign group B4Ukraine, just eight of the 248 German companies that had subsidiaries in Russia at the start of the war have cut ties with the country.

For every $4 (£3.16) in aid that Germany pledges to Ukraine, German companies pay $1 in taxes in Russia, the campaign group states.