The US on Wednesday accused Syria of buying oil from the Islamist militants of Isis and imposed sanctions on a Syrian businessman it claimed is at the centre of the trade.
The US Treasury department also announced sanctions on Cypriot and Russian businessmen who they allege have helped Syria evade international sanctions, including Kirsan Ilyumzhinov, the head of the world chess federation.
While US officials have often claimed that the Syrian government purchases oil from Isis, Wednesday's announcement represents the most direct accusation yet by the administration about the links between the Syrian regime and Isis.
"The United States will continue targeting the finances of all those enabling Assad to continue inflicting violence on the Syrian people," said Adam Szubin, acting under secretary for terrorism and financial intelligence, in a statement.
The four individuals and six entities listed by the Treasury will have any assets in the US frozen and will be barred from doing business with US companies or individuals.
The administration has been criticised for doing too little to go after Isis's oil infrastructure. However, the sanctions follow three weeks of stepped up air strikes against oilfields and trucks used by smugglers to transport the oil.
They were also announced the day before a visit to Moscow by French President François Hollande when he is expected to discuss international co-operation in the campaign against Isis, including efforts to damage the group's finances.
The targets of the new sanctions include George Haswani, a Syrian national, and his company, Hesco Engineering and Construction. The Treasury said Mr Haswani, who was placed on an EU sanctions list earlier this year, "serves as a middleman for oil purchases by the Syrian regime from Isis". It said that Hesco operated oil production facilities in Syria "reportedly in areas controlled by Isis".
Mr Haswani, who could not be immediately reached on Wednesday for comment, has dismissed the similar European accusations as "fantasies".
The Treasury also sanctioned a group of Russian and Cypriot businessmen and companies for allegedly helping the Syrian central bank evade international sanctions.
For Mr Ilyumzhinov, the alleged financial association with the government of Syrian president Bashar al-Assad adds to the eccentric Russian millionaire's already eclectic array of connections — from the Dalai Lama to aliens.
Mr Ilyumzhinov, a soft-spoken chess fanatic with a penchant for Rolls-Royces, was governor of the tiny Buddhist republic of Kalmykia until 2010 and has led the world chess federation (Fide) for two decades.
However, he is best known for his belief in aliens — he has repeatedly recounted an instance when he was abducted in 1997 by "people in yellow spacesuits".
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The Treasury on Wednesday said that Mr Ilyumzhinov had been added to its sanctions list for "materially assisting and acting for or on behalf of" the government of Syria and the Syrian central bank, as well as two Syrian central bank officials.
It did not give further details of the nature of the support, but it pointed to his links with Mudalal Khuri, who the Treasury said "represents [the Syrian] regime business and financial interests in Russia". It also placed under sanctions Russian Financial Alliance Bank, in which Mr Ilyumzhinov is a shareholder.
Mr Ilyumzhinov has a diverse business empire, stretching from sugar to banking, and a network of contacts to match. He regularly meets the Dalai Lama, and he played chess with Libyan president Muammer Gaddafi shortly before his overthrow.
On Wednesday he reacted with surprise to the Treasury's announcement. "I've visited Syria several times, I have met president Bashar al-Assad and other officials. At one time I used to play chess with Saddam Hussein. In chess there is no politics, we are promoting our sport in all countries and we talk to everyone," he told R-Sport, a Russian sports news agency.
"As far as the US Treasury is concerned, they are arresting foreign bank accounts, but I don't have a foreign bank account, or flats, or yachts — nothing."
He said he would be in the US next week for talks for his next chess world championship match, and had no plans to cancel his trip.