Canadian Prime Minister Justin Trudeau said on Sunday that mass graves found in Ukraine were evidence of Russia's war crimes and that full accountability for its actions was needed.

Trudeau, in London for the funeral of Queen Elizabeth, told reporters that he had met with British Prime Minister Liz Truss and that the Russian invasion of Ukraine was at the top of their agenda.

"Obviously the UK and Canada have been two of the strongest countries in standing up in support of Ukraine and pushing back against Russia's illegal actions," Trudeau said.

Those actions "increasingly, clearly include war crimes, include absolutely unacceptable crimes, whether we think of what we found in Bucha or the discovery of mass graves in the reclaimed territories by Ukraine," he said.

Ukrainian officials said last week that they had found 440 bodies in the woodlands near Izium in northeastern Ukraine, a town recaptured by Ukrainian forces. They said most of the dead were civilians and the causes of death had not been established. The Kremlin has not commented on the discovery of the graves, but Moscow had repeatedly denied deliberately attacking civilians or committing atrocities.

"There needs to be a proper investigation and transparency and Vladimir Putin, his supporters and the Russian military need to be held to account for the atrocities they have and are continuing to commit in Ukraine," Trudeau said.

Trudeau, who was due to meet Ukraine's Prime Minister Denys Shmyhal on Sunday evening, said Canada strongly supported Ukraine and would continue to provide aid.

Trudeau said his talks with Truss also touched on trade relations between their countries.

They discussed a Canada-UK trade deal that is being negotiated and advancing "well," Trudeau said, as well as Canada's support for Britain to potentially join the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP). (Reporting by Chris Gallagher in Washington, Editing by William Maclean and Grant McCool)