It has taken some years, but finally Turkey’s links with Islamic State are coming under sustained international scrutiny.

It might have been predictable that in the aftermath of the shooting down of a Russian jet by Turkey, the Russians would hurl accusations against the regime of Recep Erdogan — which in recent days has jailed journalists for exposing the links between the regime and Syrian Islamist rebels. Vladimir Putin referred to the Erdogan regime as “terrorist accomplices”, while Russian Foreign Minister Sergei Lavrov referred to the Turks as “co-operating with all kinds of extremist groups”.

But now the United States has demanded that Turkey act to close its porous “Gateway to Jihad” border with Syria, which has for years operated as a transit point for Islamist militants and Islamic State supporters — as well as refugees fleeing the civil war coming the other way. The Turks, under international pressure, have been closing off the border over the last year, but have insisted it would take too many troops to completely close the last stretch of border through which militants are moving.

As The Independent notes, the hollowness of this claim was illustrated by the fact that the Turks kept open a key border crossing at Tal Abyad while Islamic State controlled the territory on the other side, but promptly found the forces to close it when Kurdish forces drove Islamic State out earlier this year. Kurdish forces now control much of the border with Turkey.

Not merely does the border enable would-be jihadists and Western recruits to reach IS-controlled areas, but Turkey allows IS fighters to come back across the border and be treated in Turkish hospitals before returning to the fray; in October, Turkey’s national intelligence chief affirmed that Turkey would continue to treat IS militants.

That’s only one aspect of the regime’s support for Islamic State. Last year, a Turkish newspaper published claims IS was operating recruitment officers at two locations in Turkey, including Istanbul. Questions have been asked in Turkish parliament about the operation of IS training camps and religious gatherings in Turkey. There is extensive evidence from Turkish media outlets like Cumhuriyet (whose journalist and editor have been jailed) that Turkey is not merely providing weapons to other radical Islamist militants in Syria — including those associated with al-Qaeda — but to IS as well.

The Huffington Post recently collated the extensive collection of evidence about Turkey’s support for Islamic State.

IS and other Islamist militants aren’t merely useful allies for Erdogan’s determination to oust the Assad regime. Sections of the Turkish government view IS as a legitimate entity. “ISIS is a reality and we have to accept that we cannot eradicate a well-organised and popular establishment such as the Islamic State; therefore I urge my Western colleagues to revise their mindset about Islamic political currents, put aside their cynical mentality and thwart Vladimir Putin’s plans to crush Syrian Islamist revolutionaries,” Turkey’s intelligence chief was reported recently as saying — and he called for IS to be allowed to set up a consulate in Istanbul.

The idea of legitimising IS with diplomatic recognition demonstrates just how far removed Islamist sections of the Erdogan regime are from Western  policy regarding IS.

Beyond ideology and Erdogan’s regional ambitions — some accuse him of harbouring fantasies to re-establish the Ottoman Empire — there are more practical considerations that override even the regime’s implacable hostility to Kurds: money. Not merely does Turkey act as a conduit for oil from Kurdish-controlled Iraq, which amounts to over 600,000 barrels a day, but there are repeated claims, and considerable evidence, that IS-controlled oil has also flowed through Turkey via organised crime, at least until last year, when the United States pressured Turkey to cut off the flow of funding.

Yesterday Putin went further in Paris, saying Russia now had “information” that Turkey is acting as a conduit for IS oil “on an industrial scale” and that that was the reason why his jet was shot down. Erdogan responded by promising to resign if there was proof Turkey “buys oil from terrorists”.

Even if Turkey is somehow free of IS-controlled oil, that is only one aspect of its continuing support for IS.