Fighters from the Wagner Group, a Kremlin-linked private military company, have been supporting the Malian army in its fight against Islamist insurgents since late last year.
On Monday, German and British U.N. troops observed two planes at Gao airport, an Embraer 314 Super Tucano and an L-39 Albatros, Germany's joint military operations command said in a document seen by Reuters.
The letter, addressed to the parliament's defence and foreign committees and first reported on by Spiegel magazine, went on to point out that Russia had handed over L-39 ground attack aircraft to the Malian forces only last week.
"Two hours later..., 20 to 30 persons in military uniforms not belonging to Mali's forces were seen unloading equipment from a Malian transport plane," the document said.
"They were almost certainly members of the Russian security forces. It has to be assumed that the L-39 has to be operated by Russian forces as the Malian security forces are not up to it," the letter said.
According to the document, it was not immediately clear what role the Russians forces were supposed to fulfil in Gao.
German operations were not affected so far, the letter said.
"With the deployment of Russian forces and Russia providing high-value capabilities (ground attack aircraft) at Gao airport, the Malian forces are continuing to expand their area of operations with Russia's support towards the northeast," the military said in the document.
Berlin's participation in the U.N. peacekeeping mission in Mali has been controversial for some time in Germany as the West African country deepens its Soviet-era ties with Russia.
Mali is struggling to stem an Islamist insurgency that took root after a 2012 uprising and has since spread to neighbouring countries, killing thousands and displacing millions across West Africa's Sahel region.
France announced in February it was pulling its troops out of Mali after nearly a decade fighting insurgents there
