Vehicles apparently used to transport soldiers' bodies have been seen crossing the Russian-Ukrainian border, monitors from Europe's security body have said, reports GHN based on BBC.
The OSCE monitors said in one case a vehicle marked "Cargo 200" - Russia's military code for soldiers killed in action - crossed from Russia into Ukraine on Tuesday and later returned.
Ukraine and the West accuse Russia of sending its soldiers to fight with separatist rebels in eastern Ukraine. The Kremlin denies the allegations.
More than 4,000 people have died in the conflict between Ukraine's military and pro-Russian rebels in the eastern Donetsk and Luhansk regions.
Nearly a million people have fled their homes since the fighting started in April, a month after Russia annexed Ukraine's southern Crimea peninsula.
The violence in the east has continued despite a ceasefire deal struck in Minsk, Belarus, in September, with both sides accusing each other of shelling and other violations of the agreement.
In its weekly report on Wednesday, monitors from the Organization for Security and Co-operation in Europe (OSCE) said the "Cargo 200" vehicle was seen crossing from Russia into Ukraine at the Donetsk border checkpoint, Rostov region, on 11 November.
They said it returned to Russia several hours later. The OSCE did not inspect the vehicle.
The border crossing on the Ukrainian side - Dovzhansky - is currently controlled by the separatist rebels.
In Kiev, Ukrainian security spokesman Andriy Lysenko said that five vehicles "belonging to the Rostov funeral service" had crossed the border on Tuesday.
He alleged that they had transported "Russian military men".
Ukraine has repeatedly stated that a number of Russia's regularly troops have been killed in fighting in the Donetsk and Luhansk regions.
Moscow denies the allegations as propaganda, but admits that what it describes as "Russian volunteers" are fighting alongside the rebels.