Royal Dutch Shell has asked ship owners exporting its Nigerian oil to sign a "letter of comfort" (LoC) to guarantee that its oil is not stolen, an email from the company seen by Reuters showed.

"Please be informed we expect LPG & Products ship owners to sign the NNPC LoC for any future Shell loading voyages," the email stated.

NNPC, Nigeria's state oil company, has lifted a ban on over 100 vessels due to suspected oil theft, but asked ship owners last month to sign a letter of comfort to "guarantee to indemnify" it against any illicit use of their vessel.

This led some owners to reject pending bookings.

Shell had no immediate comment.