The Niger Delta Avengers militant group, which has mounted a bombing campaign against oil pipelines, on Friday threatened "something big" - but also wished Nigerian youngsters a Happy Children's Day.
The Avengers say oil firms in the Delta are responsible for pollution and say the poor swampland region fails to reap any benefit from the wealth on which it sits.
The militants, whose activities have hammered Nigeria's crude output, posted a warning on Twitter to the army and oil firms: "Watch out something big is about to happen and it will shock the whole world ".
They also sent out salutations to children. The Avengers
website showed a picture of children clambering over rusting oil pipelines above a message condeming the Nigerian government for denying the nation's youth the "enchanting vista" of childhood.
Children's Day is celebrated on May 27 in Nigeria, with primary and secondary schools closed.
(Reporting by Ed Cropley; Editing by Angus MacSwan)