Locked in a safe in Russia's state archive lie two white cardboard boxes holding a few fragments of darkened bones, each numbered and stored in a plastic bag.

Geneticists, forensic experts and investigators have long been certain who these remains belonged to -- Alexei, the 13-year-old son of the last tsar Nicholas II, and his sister Maria, who were shot along with their family by the Bolsheviks in 1918.

But despite DNA evidence of their identity, objections from the country's powerful Orthodox Church mean the bones remain unburied almost a century after the brutal slaying.

To satisfy them, investigators reopened the tombs of Nicholas II and his wife Alexandra, and are set to exhume Nicholas's father Alexander III.

The first results of the new tests are coming back once again confirming their identity and experts say they are struggling to see what further objections the Church could have.

But the Church is still hesitant to recognise the DNA evidence and argues that although it is willing to make the historic move, it must "rule out the possibility of any mistake whatsoever" and conduct its own research.

For experts working on the remains of the last royal family, any recognition will not come before time.

The remains of Alexei and Maria were found together in 2007 in Yekaterinburg -- the central Russian city where the last tsar, his family and their servants were massacred.

That discovery came some 16 years after the rest of the family were found in another grave, and nearly a decade after the remains of the tsar and his other three children were buried at a ceremony overseen by then-president Boris Yeltsin.

"I announce with absolute responsibility that enough evidence has been collected to prove that in the graves found in 2007 were the remains of Grand Duchess Maria Nikolayevna and Tsarevich Alexei Nikolayevich," Sergei Mironenko, the head of the state archive, told AFP in his Moscow office.

"To be honest, if you ask me, I don't understand the position of the Russian Orthodox Church."

After all this time, those seeking a burial for Alexei and Maria -- including descendants of the family -- say they want the remains to finally leave their limbo in the state archive.

"The Russian government have accepted they have not done what is supposed to be done," Paul Kulikovsky, great-grandson of Nicholas's II's sister, told AFP.

"I'm sure now that we are seeing now is just a process towards that... actually the funeral will take place," said Kulikovsky.

"Now when the Church is participating, it's a different story."