The Russian foreign ministry has accused international monitors in conflict-racked eastern Ukraine of bias towards the government.
In a statement, the ministry said it was concerned at statements by the Organization for Security and Cooperation in Europe (OSCE).
It said the OSCE reported rebel troop movements while ignoring ceasefire violations by Ukrainian forces.
Nato says convoys spotted by the OSCE this week were actually Russian.
The Kremlin denied that accusation although it has suggested in the past that Russian "volunteers" may be fighting on the rebel side.
Nato Secretary General Jens Stoltenberg has warned that any significant military build-up in eastern Ukraine would be "a severe threat to the ceasefire" agreed in Minsk, Belarus, on 5 September.
Since the ceasefire, hundreds of people have been killed in attacks by both sides. The seven-month conflict has claimed at least 4,000 lives and forced around one million people to flee their homes.
"Some aspects of the recent work of the OSCE Special Monitoring Mission to Ukraine are a cause for concern," the Russian foreign ministry said in its statement (in Russian).
"We get the impression that its efforts are directed at helping and supporting only one side in the conflict, the official authorities in Kiev... Such policies from the mission's leadership undermine trust in its work."
On Thursday, the OSCE reported that vehicles apparently used to transport soldiers' bodies had been seen crossing the Russian-Ukrainian border.
The OSCE, of which Russia is a member, has 250 unarmed civilian monitors deployed across Ukraine under its Special Monitoring Mission.