President Recep Tayyip Erdogan and other top Turkish leaders are directly involved in the illegal oil trade with the Islamic State militant group, Russian Deputy Defence Minister Anatoly Antonov charged Wednesday.

Turkey is the main buyer of Islamic State oil from Syria and Iraq, Antonov said in a nationally televised press conference.

"According to the obtained evidence, the country's supreme political leadership, including President Erdogan and his family, are involved in this criminal business," the minister said.

Erdogan on Monday denied his country was buying oil from the militant group and pledged to resign if the Russian accusation was proven true.

Oil sales are a key source of revenue for Islamic State, which controls large areas in both Syria and Iraq. The Russian Defence Ministry estimated the group's revenues from oil sales at 2 billion dollars per year.

Shortly before Antonov made his charges, US Secretary of State John Kerry said Turkey is committed to sealing its border along areas in Syria controlled by Islamic State. He stressed that Ankara has a vested interest in severing smuggling routes used by the extremist group.

"There is a 98-kilometre section of the border remaining to be closed," Kerry said while attending a meeting of NATO foreign ministers in Brussels, adding that Erdogan was in "complete agreement" with a plan to seal the area off.

"President Erdogan is committed to doing that," Kerry said.

Turkey, a NATO member, is also part of a US-led coalition battling Islamic State although it has faced sharp criticism in recent years that it had been reluctant to crack down on the extremist group and turned a blind eye to jihadists heading to Syria.

Reports in the past from Turkey's border with Syria have repeatedly said that Islamic State was smuggling oil to underground dealers in Turkey. Ankara has since stepped up its border security.

Russian President Vladimir Putin has accused Turkey of downing a Russian jet last week over the Turkish-Syrian border to protect those oil lines, a charge Erdogan rejected, saying Turkey does not deal with Islamic State.

Relations between the two countries are in a tailspin. Moscow has slapped economic sanctions on Turkey, and the United States is keen to see a de-escalation and efforts focused on battling Islamic State.

"It's as much in the interest of Turkey to close off the movement of illegally transported oil or to close off the passage of foreign fighters in one direction or another, and I'm confident Turkey understands how important it's going to be," Kerry said.

Russia identified three main oil supply routes between Islamic State territory and Turkey using satellite images, said Lieutenant General Sergei Rudskoi of the Russian general staff. Oil tanker trucks were allowed to cross the Turkish border unchecked by Turkish authorities, he said.

Russia is a staunch backer of Syrian President Bashar al-Assad and has been launching airstrikes in the country since September to support his government.

Much of Moscow's air power has targeted rebel groups battling al-Assad's forces, including militias backed by the US and Turkey.

The US is at the forefront of diplomatic efforts to resolve the crisis in Syria. World powers met in Vienna last month and agreed on a road map for a transition in Syria through a political process. The programme is still in its early stages.

"Our hope and prayer is that indeed Russia and Iran will stay committed to this concept of a transition that relegitimizes governance in Syria in a way that can bring about a ceasefire and in a way that can bring an end to the conflict," Kerry said.

Rebel and opposition groups are set to meet this month in Saudi Arabia to work out their positions.

Hardline groups are strongly opposed to al-Assad remaining in power while some moderate militias might accept him being part of a transition, creating uncertainty about the outcome of the Saudi conference.

Moreover, al-Qaeda's wing in Syria, al-Nusra Front, remains a powerful player in the north of the country and is not part of the talks. It supports strict Islamic law, opposes a democratic government and rejects any transition with al-Assad.

"If one cannot achieve that transition, let me be clear: The war will not end," Kerry said while stressing the US wants to see al-Assad removed but the government could be part of the path forward.

The Syrian civil war is estimated to have killed about 250,000 people since 2011 with many analysts skeptical that any deal can be reached, given the numerous interests of foreign powers and the divergent positions of the actors on the ground.

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