BERLIN, Germany -- The websites of German airports, public administration bodies and financial sector organisations have been hit by cyberattacks instigated by a Russian "hacker group", authorities said Thursday.

The Federal Cyber Security Authority (BSI) had "knowledge of DDoS attacks against targets in Germany", a spokesman told AFP.

A distributed denial-of-service (DDoS) attack is designed to overwhelm the target with a flood of internet traffic, preventing the system from functioning normally.

The attacks were aimed "in particular at the websites of airports", as well as some "targets in the financial sector" and "the websites of federal and state administrations", the spokesman said.

The attack had been "announced by the Russian hacker group Killnet", the BSI spokesman said.

The group's call to arms was in response to Chancellor Olaf Scholz's announcement Wednesday that Germany would send Leopard 2 tanks to Ukraine to help repel the Russian invasion, according to financial daily Handelsblatt.

Attributing Thursday's attacks directly to the hacker group, however, was "particularly hard", the BSI spokesman said.

"They call for action and then a lot of people take part," he said.

The attacks made "some websites unavailable", the BSI said, without there being "any indication of direct impacts on (the organisations') services".

Attacks on public administrations were "largely repelled with no serious impacts", the BSI said.

The interior ministry for southwestern Baden-Wuerttemberg state acknowledged "nationwide" DDoS attacks since Wednesday evening against websites, including those of public administration and the regional police.

Germany is on high alert for cyberattacks in the wake of Russia's war in Ukraine.

The Federal Office for Information Security said in October that the threat level for hacking attacks and other cybercrime activities was higher "than ever".