On Tuesday two oil flowstations operated by Royal Dutch Shell Nigeria's Niger Delta were attacked by gunmen three security sources said, Reuters reported.

It was not yet clear whether any oil production was affected by the attack in Nembe Creek in Bayelsa state.

The military and Shell officials were not immediately available for comment.

Nigeria' s oil output has been cut by a fifth in recent years due to the violence in the Niger Delta, the heart of Nigeria's oil and gas sector.

The most prominent militant group in Nigeria, the Movement for the Emancipation of the Niger Delta ended a five-month-old ceasefire last month but despite this threatened to renew attacks against the oil sector.