Nigerian militants have threatened to carry out more kidnappings in the country's oil-rich Niger Delta region, despite an ongoing military crackdown.

The region's main militant group, the Movement for the Emancipation of the Niger Delta (MEND), said in an email Friday its fighters have been instructed to attack oil facilities and kidnap people.

The group also said it killed 10 soldiers and injured at least 17 in an ambush of military gunboats preparing to raid a militant camp Thursday.

Friday's warning came as Royal Dutch Shell said one of its pipelines in the Niger Delta had been damaged.  There was no immediate evidence militants had carried out the attack.

Earlier this week, Nigerian security forces rescued 19 hostages in a series of raids in the Niger Delta.

Security sources say the hostages freed included two Americans, two Frenchmen, two Indonesians and one Canadian who were taken last week from a rig operated by the British-based company Afren.

The other 12 are Nigerians, eight of whom were kidnapped from an Exxon Mobil platform on Sunday.  The other four are employees of construction firm Julius Berger.

Nigeria's military warned Niger Delta residents last Saturday to leave areas where the militants hide.

Some information for this report was provided by AP and Reuters.