Russian President Vladimir Putin has accused Ankara of shooting down a Russian warplane to protect supplies of oil from the Islamic State of Iraq and the Levant (ISIL) group to Turkey.
On the sidelines of a global climate change conference near Paris on Monday, Putin said the downing of the plane was a "huge mistake".
"We have every reason to think that the decision to shoot down our plane was dictated by the desire to protect the oil supply lines to Turkish territory," the Russian president said.
"We have received additional information which unfortunately confirms that this oil, produced in areas controlled by [ISIL] and other terrorist organisations, is transported on an industrial scale to Turkey."
Monday was not the first time that Putin has claimed that Turkey buys oil from ISIL.
Last Thursday, the Russian leader said there was "no doubt" oil from "terrorist-controlled" territory in Syria was making its way across the border into Turkey - a claim immediately denied by Turkish President Recep Tayyip Erdogan.
Putin's strongly worded statement came hours after Turkish Prime Minister Ahmet Davutoglu again refused to apologise for the downing of the plane near the Syrian border last Tuesday.
Moscow and Ankara have been at loggerheads over last Tuesday's incident when Turkish jets shot down a Russian warplane near the Syrian border.
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Turkey has released a number of recordings which it claims prove that the warplane was warned repeatedly prior to being shot down.
Turkey has been backed by its allies NATO and the United States, with the US state department reiterating on Monday that its data "corroborated" Turkey's version of events.
But Russia has hit back hard, slapping Turkey with a series of sanctions over the weekend - including bans on Turks' labour contract extensions, chartered flights from Russia to Turkey and tourism packages to Turkey.
Despite the sanctions, however, Davutoglu said on Monday that Turkey would not apologise for "protecting its borders".
"No country should ask us to apologise," Davutoglu told reporters following a meeting with NATO's secretary-general at alliance headquarters in Brussels.
"The protection of our land borders, our airspace, is not only a right, it is a duty," he said. "We apologise for committing mistakes, not for doing our duty."
RELATED: Russia hits back at Turkey by changing Syria 'game'
Moscow's surprise intervention in the nearly five-year-old Syrian civil war in September wrong-footed the West and put Turkey, which shares a long border with Syria, directly at odds with Russian support for Syrian President Bashar al-Assad's regime.
The downing of the Russian warplane has wrecked both Turkish-Russian relations and the French-led diplomatic effort to bring Moscow closer into the fold of nations seeking to destroy ISIL through military action in Syria.
While Russia says it is targeting ISIL, many of its air strikes have been against other Assad opponents, including groups actively supported by Turkey.
Source: Al Jazeera and agencies