Questions remained on Thursday over the military plane crash that Russia said had killed dozens of captured Ukrainian soldiers ahead of a planned prisoner exchange.

The Russian defence and foreign ministries blamed Ukrainian forces for downing the IL-76 transport plane over the southern Belgorod region.

They said 65 captured Ukrainian soldiers had been on board, as well as their escorts and the crew.

Videos on social media on Wednesday showed a large plane in the region plummeting from the sky on its side before crashing in a fireball, in what the Kremlin called a "monstrous act."

On Thursday, Ukraine's SBU security service announced it had opened a criminal probe into the downing, specifically into "violations of the laws and customs of war."

And Ukraine's rights Ombudsman Dmytro Lubinets called on Thursday for the United Nations and the International Committee of the Red Cross (ICRC) to be allowed to inspect the crash site.

The ICRC did not respond to Kyiv's call, but it described the reports of the crash as "worrisome."

"We will not make any comments or speculations at this stage, until facts are established," it added.

In Paris, foreign ministry spokesman Christophe Lemoine also said France was not yet in a position to comment on Moscow's allegations.

"Russia has accustomed us to lying on these matters," he said during a press briefing on Thursday.

The UN Security Council is due to meet later on Thursday to discuss Moscow's charges that Kyiv shot down the military transport plane.

The French presidency of the Council said in a statement that the meeting, requested by Russia, would take place on Thursday at 5:00 pm (2200 GMT).

While officials in Kyiv have not denied the Russian allegations outright, they have questioned key aspects of its narrative – such as whether its servicemen were killed.

Ukraine's military intelligence said it had no "comprehensive information" detailing who was on the flight.

Kyiv has confirmed that an exchange had been scheduled for later on Wednesday on the border between the two countries.

But the military intelligence unit said Moscow had not informed it in advance that the POWs would be transported by plane, as it had in the past.

In another carefully worded statement, Ukraine's army pointed to heightened Russian military activity in the Belgorod region, pledging to continue attacking Russian military targets – again without specifically addressing Moscow's claims.

Local Ukrainian media initially cited defence sources saying that the Ukrainian army had downed the plane, and that it had been carrying missiles. That claim was quickly retracted.

President Volodymyr Zelenskyy has neither confirmed nor denied Moscow's claims.

Russia was "playing with the lives of Ukrainian prisoners," he said late on Wednesday, calling for an international investigation.

Russian officials rolled out a series of statements presenting its side of the story on Wednesday but has yet to distribute video or images of victims' remains.

State media reported the plane's black boxes were recovered but there has been no official comment their contents. (AFP)