"We are speaking about increasing inflow of (unofficial) fighters and increasing evidence that there are also some (Russian) servicemen involved in fighting," Assistant Secretary-General for Human Rights Ivan Simonovic told a news conference in Geneva, Reuters reported.
Russia denies Western accusations that it is backing pro-Russian rebels with arms and troops.
On May 21, UN officials interviewed two Russians – Captain Yevgeny Yerofeyev and Sergeant Alexander Aleksandrov – captured in eastern Ukraine. The two men believe they should be treated as captured servicemen, but Russia says they are former soldiers who had left the military.
"It is very difficult to prove whether they are servicemen or not. That is why mostly we are speaking about 'fighters' of the Russian federation," Reuters quoted Armen Harutyunyan, head of the UN Human Rights Monitoring Mission in Ukraine, as saying.
If Moscow is proven to be a party to the war, it would drag Russia into allegations of war crimes and potentially trials at the International Criminal Court, which Ukraine's foreign minister has said Kyiv wants to join.
Whether or not to declare Russia a party to the conflict is the domain of the International Committee of the Red Cross (ICRC), which is the guardian of the Geneva Conventions setting down the rules of war, and considered a reference in the United Nations.
Last July it declared the war to be a civil war, a "non-international armed conflict." The ICRC has not announced any new designation, and ICRC spokeswoman Jennifer Tobias declined to say at Reuter's request if its advice had changed in private.
