(Reuters) - The Kremlin said on Wednesday that Poland's decision to rename the Russian city of Kaliningrad in its official documents was a "hostile act", as bilateral ties continue to fray over the war in Ukraine.

Kaliningrad was known by the German name of Koenigsberg until after World War II, when it was annexed by the Soviet Union and renamed to honour Soviet politician Mikhail Kalinin.

Warsaw said on Tuesday that Kalinin's connection to the 1940 Katyn massacre - when thousands of Polish military officers were executed by Soviet forces - had negative connotations and that the city should now be referred to as Krolewiec, its name when it was ruled by the Kingdom of Poland in the 15th and 16th centuries.

"The current Russian name of this city is an artificial baptism unrelated to either the city or the region," Poland's committee on geographical standardisation said.

Kremlin spokesman Dmitry Peskov said the decision "bordered on madness".

"We know that throughout history Poland has slipped from time to time into this madness of hatred towards Russians," he told a daily news briefing.

Relations between Poland and Russia have historically often been very strained, including during and after World War Two.

Moscow says it liberated Poland when its forces drove out Nazi German forces at the end of the war. Most Poles believe the Soviet Union replaced Nazi occupation with another form of repression.

More recently, Poland, a member of the NATO military alliance, has strongly backed Ukraine after Russia's invasion on Feb. 24, 2022, and has stepped up the demolition of memorials to fallen Soviet troops across the country.

(Reporting by Reuters; Editing by Gareth Jones)