MOSCOW: Prominent Russian nationalist Igor Girkin failed in an appeal on Tuesday against his pre-trial detention on charges of inciting extremism, a Moscow court said.

Girkin, also known as Strelkov, has fiercely criticised the way that Russia has conducted the war in Ukraine. He is best known in the West for having been convicted by a Dutch court over the shooting down of a Malaysian passenger plane with the loss of 298 lives over eastern Ukraine in 2014.

His case is being closely watched as an indication of how far the Kremlin will tolerate aggressive criticism of its war effort in Ukraine, where Russia has failed to advance for months after being forced to retreat from large areas it seized in the conflict's early months.

Yevgeny Prigozhin, the Wagner mercenary leader who had conducted a running feud with the defence minister and top military command, was killed in an unexplained plane crash on Aug. 23. The Kremlin has rejected as an "absolute lie" suggestions in the West that Putin ordered his death.

Girkin's lawyer Alexander Molokhov told Reuters last month that the case against him was based on something Girkin had written on the messaging app Telegram on May 25.

Girkin had complained that authorities in the Russian-controlled part of Ukraine's Donetsk region had failed for the previous three months to provide any support payments to families of men who had been called up to fight against Ukraine.

Girkin posted that "to have someone shot for such a thing would be too little". The lawyer told Reuters that he admitted writing the post but did not agree that it amounted to extremism or incitement.

The 52-year-old faces up to five years in prison if convicted. The court, in rejecting his appeal, confirmed an earlier ruling to remand him in custody until Sept. 18.

Girkin is a former officer of Russia's FSB security service who organised pro-Russian militias to fight Ukrainian government forces in eastern Ukraine in 2014.

A Dutch court has convicted him in absentia for his role in the shooting-down of Malaysia Airlines Flight 17 over Ukraine in July of that year. He has denied he was involved.

Girkin told Reuters in May that a major crisis was brewing in Russia so he was setting up a new political group. "We are on the cusp of very grave internal political changes of a catastrophic character," Girkin said.