Russia's ambassador to Poland on Monday backtracked on controversial weekend comments suggesting Warsaw was partly to blame for World War II that sparked outcry.

"I regret having been imprecise ... which led to erroneous interpretations regarding co-responsibility during World War II," Sergei Andreev said Monday as he left the Polish foreign ministry where he had been formally summoned.

"I had no intention of offending the Polish nation," he said.

However, he reiterated that bilateral relations were at their lowest ebb since the end of World War II amid tensions over Moscow's role in the Ukraine crisis.

Poland's foreign ministry said Monday it had made a "firm protest" to Andreev over his comments, which it insisted had no basis in established historical fact.

At the weekend, Andreev had accused Poland -- which lost the highest proportion of its population in the conflict -- of "blocking the creation of an anti-Nazi coalition", making it "co-responsible for World War II".

He also justified the Soviet invasion of Poland after Hitler and Stalin secretly agreed to divide up the country between them, as necessary to "guarantee the security of the Soviet Union".

On Monday, the ambassador reiterated an opinion he ascribed to Russian President Vladimir Putin that "Polish politics in the 1930s led to catastrophe" in 1939, the year Germany and the Soviet Union invaded Poland, sparking World War II.

"We have different ideas about our common history. One must accept this and not get mad," Andreev said.

He also paid tribute to Poland's anti-Nazi resistance and "600,000 Soviet Red Army soldiers and officers who died in the fight to liberate Poland."

Warsaw reacted furiously to the ambassador's comments, saying they challenged "historical truth" and were highly damaging to relations between the neighbours, already strained by the conflict in Ukraine.

Up to six million Poles -- more than three million of them Jews -- were killed in WWII, more than a sixth of the country's pre-war population.

More than 200,000 Polish soldiers were sent to camps in Siberia after the Red Army invaded Poland in September 1939, with nearly 22,000 of their officers massacred mostly at Katyn.

Moscow blamed the Nazis for those atrocities until the fall of the Berlin Wall in 1989.