Russia is going to extraordinary lengths to justify in advance the murderous onslaught that observers fear is about to descend on Idlib, a province in north-west Syria that is home to nearly two million internally displaced people. Idlib is one of the last large populated area outside the control of Bashar al-Assad, Syria’s dictator. And Assad, backed by his Russian and Iranian allies, is determined to get it back – whatever the human cost.

In a series of coordinated moves last week, Russian government officials and military spokesmen tried to pre-empt or deflect western opposition to the expected air and ground offensive. Partly it was pure propaganda. Sergei Ryabkov, the deputy foreign minister, accused the US of plotting forcible regime change in Damascus. “Again, we are witnessing serious escalation of the situation,” he claimed.

Unfortunately perhaps, this is disingenuous fantasy. A distracted Donald Trump has shown no interest in toppling Assad. He has ended support for rebel groups and given Vladimir Putin, Russia’s president, a virtual free hand. Airstrikes by the US, France and the UK after Assad’s chemical weapons attack on Douma in April proved to be an ineffective one-off. Trump has turned his back on Syria and plans to pull out the remaining US special forces fighting Islamic State as soon as possible.

The escalation is all on Russia’s side. It is assembling a naval armada off the Syrian coast, comprising 25 ships, combat aircraft and the missile cruiser Marshal Ustinov – the biggest show of force since Putin intervened in Syria in 2015. The fleet is ostensibly engaged in exercises. But Dmitri Peskov, the Kremlin’s spokesman, admitted the drills were directly linked to Idlib, which he termed a “terrorist hotbed” that must be tackled soon.

The Russia-Syria axis is stepping up its diplomatic offensive, too. Sergei Lavrov, Russia’s foreign minister, warned last week that “militants” in Idlib (he did not say who) must be liquidated, describing them as “a festering abscess”. Walid al-Moualem, Syria’s foreign minister, who met Lavrov in Moscow the following day, was blunt: “We are at the final stage of solving the crisis in Syria and liberating our whole territory from terrorism.”

The Russian and Syrian regimes claimed to be solely concerned with fighting terrorism when defending their previous, indiscriminate missile, barrel bomb and artillery attacks on civilian residential areas, hospitals and schools, notably in Aleppo and Eastern Ghouta, which caused mass casualties. Yet according to the UN, of the three million people in the line of fire in Idlib, only about 10,000 are armed jihadists. In total, about 70,000 anti-regime rebels are cornered there.

António Guterres, the UN secretary-general, expressed concern last week that a “humanitarian catastrophe” was looming, possibly even larger in scale than those elsewhere. And he reminded Turkey and Iran, Russia’s partners in the Astana peace process, that they had jointly designated Idlib a “de-escalation zone” – meaning it should be protected.

But like all the other Russian-declared de-escalation zones, the Idlib region has already been attacked. According to the human rights advocacy group, the Syria Campaign, there has been a series of atrocities in recent weeks, including the bombing of Urem al-Kubra on 10 August that killed 39 people. Although Turkey, fearing another cross-border refugee influx, opposes any new offensive, its forces inside Syria appear powerless to prevent it.

The aftermath of an airstrike on a market in Maaret al-Numan in southern Idlib in October 2017. Photograph: AP

“To justify attacking Idlib, the regime often claims the province is full of terrorists but the truth is the vast majority of the population are civilians. An offensive is predicted to displace more than 700,000 people and create a humanitarian catastrophe for hundreds of thousands more,” a spokeswoman for the Syria Campaign said. Up to 1.6 million people in Idlib were already in need of food assistance, she added. On Friday, meanwhile, rebels reportedly dynamited bridges in the south of the province to slow regime advances.

Russia’s efforts to influence international opinion include a repeat of previous disinformation campaigns concerning chemical weapons. Despite documented evidence of numerous instances of illegal chemical weapons use by the Assad regime, Moscow and Damascus continue to insist these attacks either did not happen or were staged by jihadists or rebel factions.

Major-General Igor Konashenkov, the Russian defence ministry spokesman, recycled this fake news angle last week, claiming that fighters belonging to the main jihadist group, Hay’at Tahrir al-Sham, had smuggled eight canisters of chlorine gas into a village near Jisr al-Shughur, south-west of Idlib city. Their plan, he said, was to stage a chemical weapons atrocity and blame it on the regime, thereby inviting renewed western intervention.

The fear now, shared by the UN’s Syria envoy, Staffan de Mistura, is not so much that such obvious false flag operations will be believed, but that Assad intends to resume chlorine attacks and then claim it is all a rebel stunt. Mistura called last week for humanitarian corridors to allow civilians to escape the Idlib kill-box, warning of the “most horrific tragedy” if they remain trapped.

Putin is eager to portray the Syrian war as all but over – and he doubtless wishes it was, since his three-year campaign has proved costly in money and materiel. But his attempts to switch the international focus to post-war reconstruction – he recently discussed this with German chancellor Angela Merkel – are also designed to draw attention away from Idlib, where the war is far from won.

In order to vindicate the 2015 intervention, ensure Assad’s survival and seal an epic Russian strategic victory over the US, Putin requires physical control of Idlib – the final piece of the fractured Syrian jigsaw. His pre-emptive message to the western democracies, with axe poised to fall, is keep out and don’t interfere – whatever the cost in human life and suffering.