THE HAGUE (AFP) - Dutch investigators said on Tuesday (Sept 3) they want a Ukrainian man who is reportedly at the centre of a stalled prisoner swap with Russia to stay in Kiev so he can be questioned over the downing of flight MH17.

Former pro-Russian separatist Volodymyr Tsemakh is wanted for questioning ahead of a trial over the 2014 tragedy that is due to start in the Netherlands next year.

Russian and Ukrainian media have reported that Tsemakh's case has held up a large prisoner exchange that Kiev and Moscow, with the Kremlin demanding that he be included.

"We would like to talk to Mr Tsemakh and ask him questions, so we'd rather have him available for the investigation in Ukraine," Brechtje van de Moosdijk, a spokeswoman for the Dutch-led MH17 investigation, told AFP.

"He's now in Ukraine, in a Ukrainian prison cell and if he's being exchanged of course it's hard to say that we can still question him," van de Moosdijk added.

The Malaysia Airlines passenger plane travelling from Amsterdam to Kuala Lumpur was shot down by a Russian-made BUK missile in 2014 over eastern Ukraine, with the loss of all 298 people on board.

International investigators said in June that they are going to put three Russian nationals and one Ukrainian on trial in the Netherlands on March for the downing of MH17, although they will likely be tried in absentia.

Van de Moosdijk would not confirm reports in Ukrainian media that a Dutch prosecutor had written to Ukrainian authorities asking them not to transfer Tsemakh to Russia.

Fred Westerbeke of the Joint Investigation Team (JIT) reportedly wrote that it was "of paramount importance" that the suspect "remains available" for further questioning.

The Ukrainian secret service captured Tsemakh at his home in separatist-held eastern Ukraine in June, and he was then secretly transferred while unconscious to Kiev.

Ukrainian officials accuse Tsemakh of being part of a "terrorist organisation" - the phrase they use for separatists. His Ukrainian lawyer said on Tuesday that Kiev had further prolonged his detention.

The Kremlin has reportedly demanded that Tsemakh, a former miner who became a so-called "air defence specialist" in the Ukranian separatist forces, should be among those handed over.

Ukraine on Friday denied reports that the swap - also said to involve Ukrainian filmmaker Oleg Sentsov and 24 Ukrainian sailors - was under way.