A cargo of diesel for Libya's rebel-held ports was loaded in Malta and shipped aboard the tanker Delos for Benghazi just over a week ago, according to a Reuters report.

The report said that marine raffic showed the tanker reached Tobruk on April 17, and six days later, during which time there was no satellite information available, sailed from the port of Benghazi on April 23, heading back to Malta.

"The vessel loaded ship-to-ship out of Vitol's storage (in Malta) and moved to Tobruk over a week ago," one source said.

The fuel delivery was thought to be part of an agreement to swap crude oil in exchange for products.

Vitol fixed a gasoline cargo to Benghazi earlier this month but it was not clear if it arrived.

The Delos has a capacity of around 43,000 metric tons.

The United States this week took steps to boost aid to the Libyan opposition council and to approve oil exports made under its auspices, creating a loophole in US sanctions that could mean millions of dollars in revenue for rebels.