* President Jonathan leaves after first trip to northeast
* Seven blasts detonated in Maiduguri after he departs
* Boko Haram main threat to Africa's top oil producer
Seven loud explosions shook Nigeria's northeastern city of Maiduguri on Friday, witnesses said, hours after President Goodluck Jonathan ended a trip there to try to galvanise support for his battle against Islamist insurgents.
Witnesses said the explosions happened within minutes of each other, suggesting the kind of coordinated strike that used to be the hallmark of Islamist sect Boko Haram, before a military campaign weakened them. They remain the main security headache to Africa's top energy producer.
President Jonathan spent the night in Maiduguri, the epicentre of the insurgency, which seeks to carve an Islamist state out of religiously mixed Nigeria. It was his first trip to the troubled northeast since becoming president.
During it, he poured cold water on the idea of an amnesty for Islamist fighters and urged the region's tribal elders to do more to help fight the insurgents.
The blasts in the Jajeri ward of the city occurred about five hours after he left in the afternoon, witnesses said.
"I heard seven explosions successively. They were huge, but I have no way of knowing whether the explosions hurt anybody," Jajeri resident Usman Abubakar told Reuters by telephone.
There was no word on casualties and a security forces spokesman for the northeast was not immediately available for comment.
Earlier in the day Jonathan gave a speech in which he called on tribal elders to wheedle out the Islamists in their midst. Traditional leaders accuse the security forces of being too heavy handed and indiscriminate, killing dozens of civilians.
"Instead of condemning the operations of the JTF (military and police joint task force) we should work to fight the insurgency," Jonathan said, replying to the criticism.
"Unless the Elders of Borno condemn and fight Boko Haram the people of the state shall continue to suffer from their attacks," he added.
Jonathan said on Thursday he was not ready to offer an amnesty to members of Boko Haram, brushing aside a proposal from the country's most senior Muslim spiritual leader, the Sultan of Sokoto.
Attempts to establish contact with secretive organisation for negotiations last year yielded no results, and the sect's self-proclaimed leader Abubakar Shekau last week rejected the whole notion of peace talks in a video.
Some analysts say Boko Haram has become a label used by a disparate collection of armed groups spread across the north, some of which, including Shekau's, have been trained by al Qaeda-linked Islamists in Mali and Niger.
Criminal gangs have also sprung up, taking advantage of worsening security in the region. Boko Haram has regularly targeted soldiers, police, government officials and Christians.