Ukraine has criticized Germany's slow delivery of tanks following its recent successes in the north-east of the country.
"For six months there are no tanks because there has been no 'political decision' about them," presidential office adviser Mykhailo Podolyak wrote in German on Twitter on Tuesday.
Because of Germany's hesitation, he said, Russia could continue its "terror" and Ukrainians would die as a result.
"Germany, we are waiting for you to fulfil your promise," the 50-year-old wrote.
In the Ukrainian version of the Twitter post, Podolyak also directly named the so-called circular swap system operated by the German government to provide weapons indirectly to Ukraine.
"The circular swap system does not work," he said, referring to a mechanism where states from the former Eastern Bloc hand over their old stocks of Soviet weapons to Ukraine and receive modern equipment from Germany as replacements.
The Ukrainian army used, among other things, T-72 tanks supplied by Poland during its latest successful offensive in the Kharkiv region.
Russia invaded Ukraine on February 24 and subsequently took control of large parts of the east and south of the country. Since the beginning of September, however, Ukrainian troops have managed to recapture a considerable amount of territory.
There has also been criticism of Germany's delays in supplying Ukraine from within the ranks of the country's ruling coalition.
The youth wing of the pro-business Free Democrats (FDP), which is in government with the Greens and the centre-left Social Democrats (SPD), has called for the swift delivery of the armoured vehicles.
"The time for prevarication is long past," Franziska Brandmann, the federal chair of the FDP's youth organization, said on Tuesday: "Decisive action is called for now."
Brandmann called on German Chancellor Olaf Scholz, who is a member of the SPD, to show leadership. "The current successes of the Ukrainian army show anyone who loudly proclaimed that a Ukrainian victory in the Russian war of aggression was impossible was ignorant, or was even lying," she said.
