Syria is under international scrutiny because of the violent crackdown on protests against President Bashar Assad and activists are keen to shame any companies still supplying his government. 

Swiss-based trading firm AOT Trading has been a rare supplier of Russian gasoil to the increasingly isolated Syrian government.

Trade sources said that at least some of AOT's Syria-bound fuel, such as gasoil delivered aboard the tanker “Ray G” last week, was initially sourced by Swiss-based trader Gunvor, part-owned by a Russian tycoon. 

"SECO (the Swiss Secretariat for Economics)... is in contact with several companies active in oil products trading, including the companies mentioned (AOT Trading and Gunvor). Currently, SECO has no information that the oil sanctions have been violated by companies based in Switzerland," Antje Baertschi, a spokeswoman for SECO, said late on Thursday. 

AOT Trading was not immediately available for comment and Gunvor declined to comment. 

There is no embargo on supplying Syria with fuels such as the gasoil AOT delivered, which can be used both for heating and to fuel army tanks. 

But most former suppliers such as traders Vitol and Trafigura have dropped trade after Syrian state-owned oil companies, including Sytrol, formally responsible for product imports, was placed on Swiss, European Union (EU) and US blacklists. 

It was not clear who AOT Trading's counterparty was in Syria. Venezuela has delivered at least two shipments of diesel to the government of Syria. 

Syria has faced worsening shortages this winter after Western pressure made imports difficult as the country needs to bring in at least half of its diesel requirements. 

Diplomats blame power and fuel shortages on increased demand from the military, while the government says attacks on infrastructure have reduced supply. 

AOT is part of a group owned by NPM/CNP, a Belgian holding company controlled by Belgian billionaire Albert Frere. It has other stakes in a variety of firms including Iberdrola and Total. 

The Swiss government added seven names - most of them ministers - on Friday to a list of top Syrian officials on whom it imposed sanctions last year to put pressure on Assad to end his crackdown.