The pro-Russian separatist militants who control parts of eastern Ukraine and have been implicated in bringing down Malaysia Airlines flight MH17 have been banned from swearing in public by their volatile leader, Igor Strelkov.

The much-feared rebel defence chief warned his militants today that foul language is a "grave sin" that "spiritually demeans us and will lead our army to defeat".

The decree issued by Strelkov says foul language is a \"grave sin\". Twitter @dnrpress

Strelkov issued the order via his Twitter account to fighters in the eastern Ukraine region of Donetsk, who are currently facing strong advances from the Ukrainian army.

The decree reads: "We call ourselves an Orthodox Christian Army. Foul words do not have a Russian origin and were used by Russia's enemies for the desecration of holy places."

Strelkov, whose real name is Igor Girkin (Strelkov means "shooter"), warned rebels that any of them caught swearing could expect punishment for the "serious disciplinary infraction".

Shaun Walker, a Guardian journalist based in Moscow, has met with Strelkov and the Donetsk rebels over the past week. He tweeted today: "Strelkov bans swearing among the Donetsk rebels. On the basis of 99.8% of my interactions with them recently, this could prove problematic."

Today's decree comes as the Ukrainian government announced that it has retaken another town in the Donetsk region and "mopped up" pockets of resistance from fighters, who are being pushed back towards the Russian borders in the increasingly bloody operation.

A fresh round of fighting near to the site of the crashed MH17 Boeing 777 this week forced Dutch experts to cancel a planned trip to collect victims' remains for the fourth day running, because the situation is too unsafe.