Nigeria offered an amnesty yesterday to Islamist militants who surrender and said 17 people had been killed on the fifth day of a military operation to try to crush the Boko Haram insurgency in the country’s northeast.

In their biggest offensive since the insurgency began in 2009, Nigerian forces are trying to chase well-armed militants out of territory they control in remote semi-deserts around Lake Chad, along the borders with Cameroon, Chad and Niger.

They have also launched a crackdown on them in northeastern cities such as Maiduguri, where the sect has powerful cells.

Nigeria’s defence spokesman Brigadier General Chris Olukolade said the operation was continuing on Sunday, with patrols sent out to secure towns and villages, and that special forces had killed 14 insurgents in battles that left three Nigerian soldiers dead and seven wounded.

President Goodluck Jonathan declared a state of emergency on Tuesday in the states of Borno, Yobe and Adamawa. The operation has targeted areas of Africa’s top energy producer where Boko Haram, which is fighting to create a breakaway Islamic state in religiously mixed Nigeria, has bases and weapons caches.

Nigerian forces used jets and attack helicopters to bombard militant camps in the northeast on Friday.

Some analysts fear the offensive may have rendered the already slim chance of a political solution to the conflict even slimmer, but the president’s spokesman Reuben Abati denied this. Jonathan set up a committee to work out the terms of a possible amnesty for Boko Haram members last month.

“Mr President has urged Boko Haram members to surrender their arms and embrace the amnesty option which is still open as the committee is working on the option of dialogue for a peaceful resolution,” Abati said by telephone.

Phones have been largely cut to the entire northeastern region, to prevent the rebels from communicating.

“The insurgents have people who look out and tip them off by phone, which opens the military up to ambushes. Without phones, raids will have the element of surprise,” a security source said.