Commodities trader Vitol has agreed to charter two supertankers to deliver crude oil to Nigeria’s giant Dangote refinery from the U.S. in May, according to a shipping source and fixtures data this week.

Vitol is looking to charter Marshall Islands-flagged very large crude carrier Water Tiger for about $3.95 million to ship oil from the U.S. Gulf Coast to Nigeria between May 13 and May 16, a shipping source said.

It also is considering a charter of Liberia-flagged Hafeet to Nigeria from the U.S. Gulf Coast between May 15 and May 20, according to the source and fixtures contracts seen by Reuters.

Vitol has agreed to charter the vessels, but contracts are yet to be signed, the source said.

Vitol did not immediately respond to a request for comment

The agreements come as Chinese state energy major PetroChina 601857.SS has a cargo of U.S. crude that has been waiting to unload at Nigeria’s new refinery for nearly a month due to payment issues, according to four trading sources and shipping data.

Potential sellers of U.S. WTI crude to Dangote have been confronted with payment terms involving a 60 to 90 day credit or an exchange of refined products for the crude oil, three of the sources said. Credit terms for oil deals are typically 30 days.

Source: Reuters (Reporting by Arathy Somasekhar in Houston; Editing by Josie Kao)