Moscow said Monday it was expelling 40 German diplomats in response to the "unfriendly decision" by Berlin to kick out Russian diplomats over the conflict in Ukraine.

Russia's foreign ministry said in a statement it summoned Germany's ambassador in Moscow and handed him a note "declaring persona non grata 40 employees of German diplomatic institutions in Russia as part of a symmetrical response". 

"A strong protest was made to the head of the German diplomatic mission in Moscow in connection with the openly unfriendly decision of the German government," to expel Russian diplomats, the ministry said. 

Earlier in April, Germany said it was expelling 40 Russian diplomats, amid similar moves by other European states, over Ukraine.

Berlin responded to Monday's announcement with defiance, describing it as "in no way justified".

"The 40 Russian diplomats in Germany whom we expelled three weeks ago did not serve diplomacy for a single day during their stay in Germany," Foreign Minister Annalena Baerbock said in a statement.

"Rather, these people worked systematically for years against our freedom and against cohesion in our society," she said.

"Their work also threatened those who sought protection in our country. We could no longer tolerate that, nor will we tolerate such things in the future."

The German diplomats in Russia, meanwhile, "have not done anything wrong", Baerbock said. 

"Despite the increasingly adverse circumstances there, they went to Russia with openness, curiosity and great commitment to serve our bilateral relations there."

Baerbock had said earlier that Germany was expelling the Russians in response to the "unbelievable brutality" of Russian forces in its pro-Western neighbour Ukraine. 

The Russian foreign ministry on Monday called Baerbock's words "unacceptable". 

It added that Berlin's decision was "motivated by an absolutely false assertion that the work of the above-mentioned employees was aimed at undermining the 'freedom of Germany' and 'unity of German society', as well insinuations about what is happening in Ukraine".