Nigerian soldiers have killed three members of Islamist militant sect Boko Haram and arrested 25 more in the north-east, the army said yesterday, two weeks after the launch of a major ground and air offensive to crush the group's four-year rebellion.

President Goodluck Jonathan declared a state of emergency in three north-east states of Borno, Yobe and Adamawa on May 14 and ordered thousands of additional troops to the area where Boko Haram has established bases and weapons dumps.

The assault in the remote semi-deserts along the borders with Cameroon, Chad and Niger is the biggest effort yet to end fighting in which thousands have died. Security sources say soldiers from Niger and Cameroon are also involved.

Rights groups are concerned that the secrecy of the operation means soldiers may carry out human rights abuses. They point to previous evidence of the army carrying out extra-judicial killings.