A protester holds a poster that depicts Russian President Vladimir Putin and reads "Putin, killer!" during a demonstration against Russia on Friday in Istanbul. (Cagdas Erdogan/AFP via Getty Images)

The war of words between Moscow and Ankara took a new turn early this week in the wake of Turkey's Nov. 24 downing of a Russian jet that had flown briefly into Turkish airspace during a sortie in Syria.

Russian President Vladimir Putin and his Turkish counterpart, Recep Tayyip Erdogan, traded accusations on the sidelines of the Paris climate summit. Putin said Turkey's behavior was aimed at protecting its interests. Some Erdogan critics, now including Putin, allege that these involve ties to the Islamic State militant group.

"At the moment, we have received additional information confirming that oil from the deposits controlled by Islamic State militants enters Turkish territory on an industrial scale," Putin said Monday. "We have every reason to believe that the decision to down our plane was guided by a desire to ensure security of this oil’s delivery routes to ports where they are shipped in tankers."

[How the Russian-Turkish rivalry shaped the world]

The accusation was angrily rejected by Erdogan, who said he would resign if the claim could be proved.

"I will not remain in this post," he said. Then he went on the offensive: "But I am asking Mr. Putin, would you remain?" Here's how the pro-government Daily Sabah described what Erdogan said next:

He stated that a Russian-Syrian citizen has been buying oil from [the Islamic State] and then selling it to Assad regime, which he said was also confirmed by U.S. sources. "First they [Russia] should give an account of this," he added.

The president also said that Russia's claim that Turkey bought oil from [the Islamic State] terrorist organization is not 'moral', adding "such claims have to be proved."

It's not clear what incident Erdogan is referring to. Last week, the Obama administration accused the Assad regime of buying oil from the Islamic State; at the same time, the Treasury Department penalized a number of Russian and Cypriot businessmen suspected of helping the Syrian central bank evade international sanctions.

This tit-for-tat slander is part of the wider escalation of tensions between the two countries after the plane downing. Russia has slapped a range of economic sanctions on Turkey, including bans on agricultural imports and even on Russian soccer teams acquiring Turkish players.

The two governments have differing agendas in Syria. Putin initiated a Russian intervention into the war-torn country in late September. The intervention, although framed as a bid to defeat the Islamic State, appears aimed primarily at buttressing the embattled regime of Syrian President Bashar al-Assad, a longtime Moscow ally.

[Turkey's messy war in the Middle East, explained.]

Turkey under Erdogan has been outspoken in its calls for Assad's ouster, but critics say Ankara has been too lax in controlling the vast, porous border with Syria — a neglect that they say has enabled the rise of militant groups such as the Islamic State. The group is thought to have a lucrative economy based on the smuggling of oil and other contraband through Turkish networks.

In remarks also made in Paris, President Obama stressed the need for Turkey to better clamp down on its border, though he acknowledged the humanitarian aspects of Ankara's open-border policy as it accommodated desperate Syrians as well as the "enormous strains" the refugee crisis has placed on Turkish authorities. The Turkish government says it has spent about $8 billion in unbudgeted funds providing for the about 2 million Syrian refugees within its borders.

But the current geopolitical clash highlights the awkwardness and complexity of the struggle in Syria, which pits a host of non-state and regional actors against one another. Turkey belatedly took the fight to the Islamic State this year, but it has devoted equal efforts to acting against separatist Kurdish militias on both sides of its border with Syria.

Erdogan's opponents have criticized Ankara's decision to bring down the Russian jet.

"It was not right for Turkey to shoot down the plane because it escalated [tension] in a fragile situation and increased the risk of deaths among the [anti-Islamic State] set of countries," Figen Yuksekdag, the co-leader of the Peoples' Democratic Party, a leftist, pro-Kurdish party in the Turkish parliament, said in an interview with Britain's Daily Telegraph newspaper while on a trip to London.

In other remarks made on her visit, she said the Turkish government's actions "only worsen the situation in Syria."

En route to Qatar after the Paris talks, Erdogan lamented the souring of ties with his Russian counterpart. In many respects, as WorldViews noted last year, the duo hold much in common.

"Putin’s statements about me have always been about my bravery and boldness," Erdogan said. "He also had many words on my honest statesmanship."

This article has been updated.

Related in WorldViews

Why Russia is in Syria

7 Middle East crises that are a bigger problem than Iran's nuclear program

When the West backed Islam to defeat Russia

Was Putin right about Syria?