The European Court of Human Rights ruled on Monday it was incompetent to judge whether Russia's investigation into the 1940 Katyn killings of Polish war prisoners was insufficient.

An estimated 22,000 Polish military officers and intellectuals were killed Katyn, western Russia, many of them trucked in from prison camps, shot in the head from behind, and shoved into mass graves.

After blaming Nazi Germany for the Katyn massacre for decades, the Soviet Union admitted in April 1990 that its forces were responsible. But none of the culprits has ever been identified and investigations have been shelved.

Relatives of those murdered by the Soviet secret police during World War II claimed that the 1990-2004 investigation by Russian authorities into the massacre was "inadequate".

But the court said it lacked jurisdiction to judge on the matter because the Katyn massacre took place before the adoption of the European Convention on Human Rights in 1950.

"I can only say that we are rather disappointed by this verdict," Deputy Foreign Minister Artur Nowak-Far said in a statement.

"The ruling does not take into account all the arguments of the Polish side that have here a great moral and historic right," he added.

The Katyn killings have been casting a shadow over relations between Poland and Russia for many years now, with Warsaw especially disappointed that Moscow was not willing to transfer documents concerning the massacre for further analysis.

The European court also agreed that Russia had failed in this regard by refusing to submit a key procedural decision from the investigation and offered no substantive reason for maintaining its classified status.

Germany invaded Poland from the west in 1939 and Soviet forces occupied the eastern half of Poland. As a result, tens of thousands of Polish military personnel fell into Soviet hands and were interned in prison camps inside the Soviet Union.

On April 13, 1943, the Germans said they had found the mass graves of Polish officers in Katyn forest near Smolensk.