© IA FrontNewsRelatives said the boy was just "torn apart"
Vladik Shikhov was blown up outside his home in the Aleksandrovskoye village in the breakaway Donetsk region of eastern Ukraine, which has been held by Russia-backed rebels since 2014.
Last week Russia sent 4,000 troops and convoys of tanks to the border, prompting experts to warn the conflict could trigger a new world war in just four weeks.
© IA FrontNewsRelatives said the boy was just "torn apart"
The boy's grieving family claim he was killed in a drone attack orchestrated by the Kiev government - which they deny.
A relative said after the five-year-old's funeral: "Our child just went outside. And the explosion happened."
"He was just torn apart," another woman said.
But the Western-backed government of Ukraine insisted the drone attack was invented to justify a full-scale invasion of the rebel-held territory by Russia.
Denis Kazansky a representative of Ukraine in the Trilateral Contact Group on the crisis, claimed the boy died when an old shell exploded.
Convoys of Russian tanks and armoured vehicles is being mass transported to the border
"And after dropping an explosive device on them (the drone) just as calmly flew back.
"Who can believe such nonsense? But then a real bacchanalia began in the Russian media."
Vladik was "non-existent crucified boy" - and had been killed by a grenade or shell stored close to his home, said Kazansky.
Russia was using "this tragic incident to incite hatred and create another myth about the atrocities of the Ukrainians.
"For two days now, propagandists from the Russian Federation have been hysterical, cursing us in every way," he said.
The death certificate - shown by the boy's father Sergey Shikhov- says Vladik (Vladislav) died "as a result of mine and explosive injuries, with the traumatic amputation of limbs".
The boy's father told a journalist that when the explosion came the grandmother "was in the hallway near the door" following the boy into the yard.
"She was thrown back inside the house, she had a head injury, she was sent to intensive care."
Neighbours heard "the sound of a drone in the air" immediately before the boy died, said the father.
The child's grandmother Natalya Terentyeva, 56, who was with the boy when he died, wept: "Forgive me that I let you out at that very minute."
After the funeral, grieving relatives also hit back at claims that the boy did not exist.
Human rights observer Tatyana Montyan told the Russian media: "Due to the large number of reports that the boy allegedly did not exist, I decided to go to the funeral and see with my own eyes.
"A funeral cannot be faked. It was an ordinary funeral, people who knew the boy, all his relatives, mother, father, grandmothers - all in terrible grief."
During a call with Boris Johnson, Ukrainian president Volodymyr Zelenskyin pleaded for a larger British and NATO troop presence in the region, plus further sanctions on Vladimir Putin.
He said he "calls on the United Kingdom and other member states of the alliance to strengthen its presence in the region and increase the pressure of sanctions on the Russian Federation.
Johnson said he had "significant concerns" over the Russian military buildup and "reaffirmed his unwavering support for Ukraine's sovereignty and territorial integrity".
Ukraine and Russia have remained technically at war since 2014.
R.C.