Namibia has rejected a German offer of compensation for the mass murder of tens of thousands of indigenous people more than a century ago.

German occupiers in Namibia almost destroyed the Herero and Nama peoples between 1904 and 1908 as they consolidated their rule in the new colony in south-west Africa. Some historians have described the bloodshed as the first genocide of the 20th century.

The two countries have been discussing an agreement on an official apology from Germany and an increase in development aid, but the talks appear now to be running out of momentum.

Namibia’s president, Hage Geingob, said on Tuesday that the most recent offer “for reparations made by the German government … is not acceptable” and needed to be “revised”.

No details were provided on Berlin’s proposal, but unconfirmed media reports have referred to a sum of €10m.

The row comes at a time of broader reassessment of Africa’s colonial history and the suffering inflicted by European powers on populations across the continent. Partly inspired by the Black Lives Matter movement, there have been steps to remove monuments to colonialists that remain in many cities and to change names of streets.

Other countries in Africa are watching the negotiations between Namibia and Germany closely as they consider launching their own efforts to gain compensation for the violence and theft of decades of European rule.

Ruprecht Polenz, the German government’s special envoy for the negotiations, did not deny that his side’s offer had been rejected. “What matters is that the negotiations are ongoing, and I am still optimistic that a solution can be found,” he said. “Germany wants to live up to its moral and political responsibility.”

The German government is reluctant to use the word “reparations” in a declaration accompanying any agreement with the Namibian government because of concerns that such a statement could provide a legal blueprint for future restitution claims from Poland, Greece or Italy relating to crimes during the second world war.

A Namibian official involved in the negotiations said Germany had proposed an alternative description of cash payments as “healing the wounds”.

Polenz said: “For us this is not a legal question but a political and moral question.”

Germany was relatively late to acquire African colonies, but in 1884 as European powers scrambled to carve up the continent, Berlin moved to annex a colony on the south-west coast. Land was confiscated, livestock plundered, and native people were subjected to racially motivated violence, rape and murder.

In January 1904 the Herero people – also called the Ovaherero – rebelled. The smaller Nama tribe joined the uprising the following year.

In response, colonial rulers forced tens of thousands of Herero into the Kalahari desert, their wells poisoned and food supplies cut. Others were rounded up and placed in concentration camps. Half of the Nama population also died, many in disease-ridden death camps such as the infamous site on Shark Island, in the coastal town of Lüderitz.

Germany was forced out of the colony in 1915. The killings there are seen by some historians as important steps towards the Holocaust in Europe during the second world war. Namibia passed to South African rule, and gained independence in 1990.

Germany’s 29-year rule in a second colony, which eventually became Tanzania, was also bloody. Tens of thousands of people were starved, tortured and killed as colonial forces crushed rebellions.

Hussein Mwinyi, a Tanzanian government minister, told parliamentarians in February that officials were closely watching “steps taken by Kenya and Namibia governments in seeking reparations from Britain and German governments respectively”.

Other former colonial powers have been deeply reluctant to acknowledge the violence associated with their imperial history.

Belgium long refused to officially recognise the cost of its invasion and exploitation of the Democratic Republic of the Congo, where it is thought that about 10 million people – roughly half the population – died during its rule. Only in June did King Philippe express his “deepest regrets” for the brutality of his country’s reign over the vast, troubled state.

In 2013 the British government said it “sincerely regrets” acts of torture carried out against Kenyans fighting for liberation from colonial rule in the 1950s and 1960s. It said it would pay out £19.9m to 5,200 Kenyans who were found to have been tortured.

Officials in Berlin rejected the use of the word “genocide” to describe the killings of the Herero and Namaqua until July 2015, when the then foreign minister, Frank-Walter Steinmeier, issued a “political guideline” indicating that the massacre should be referred to as “a war crime and a genocide”.