Kyiv, Ukraine - A senior Ukrainian presidential aide on Wednesday said North Korea was helping Russia kill Ukrainian civilians and called for greater international isolation of both countries.

Russian President Vladimir Putin and North Korean leader Kim Jong Un on Wednesday signed a comprehensive strategic partnership, deepening military and political ties that have flourished since Russia invaded Ukraine.

"There is no doubt that North Korea is actively cooperating with Russia in the military sphere today and deliberately provides resources for the mass murder of Ukrainians," the aide, Mykhailo Podolyak, told AFP.

The US and its allies have accused North Korea of providing ammunition and missiles to help Russia's war, and Ukraine has reported finding fragments of North Korean munitions on the battlefield.

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Deliveries of North Korean artillery shells to Russia were helping its forces maintain a "high intensity" of attacks, Podolyak said.

"This has an impact on the course of events in the war as a whole and significantly increases the number of civilian casualties."

The aide blasted the "absolute incapacity" of international sanctions on Russia and North Korea.

"It is obvious that the restrictive measures imposed on Russia and North Korea are not enough. We need a more rigorous approach to real isolation," he said.

During a state visit to Pyongyang, Putin called for a review of sanctions on North Korea and said the two countries were together fighting "US hegemony."