LAGOS, Nigeria (AP) — U.S. Secretary of State John Kerry is encouraging the main candidates in Nigeria's upcoming presidential vote to accept the results and tamp down potential postelection violence in a country reeling from an al-Qaida linked insurgency.

After touching down Sunday in this steamy, sprawling city of 21 million, Kerry met first with President Goodluck Jonathan and later, at a different location, sat down with former military dictator Muhammadu Buhari, the loser in the 2011 race.

The Feb. 14 election in Africa's most populous country comes amid a series of killings and kidnappings carried out by Boko Haram, an al-Qaida-linked group that has seized large portions of northeast Nigeria and attacked civilians.

Last week, Boko Haram claimed responsibility for a massacre of hundreds of people in the town of Baga on the shores of Lake Chad.

In fierce fighting Sunday, Nigerian troops battled Islamic extremists who attacked Maiduguri, the biggest city in the northeast. Dozens of combatants have been killed and wounded, soldiers and hospital workers said.

Lagos, the country's commercial capital, is nearly 1,000 southwest of Maiduguri, the birthplace of Boko Haram that has been attacked many times in the long-running Islamic insurgency that killed 10,000 people last year.

Kerry intended to appeal to Jonathan and Buhari to instruct their supporters to refrain from violence, State Department officials said ahead of Kerry's trip.

Jonathan's disputed 2011 election victory triggered riots in the north that killed an estimated 800 people.

American diplomats have expressed concern about what could be a prolonged election.

Under Nigeria's election laws, a candidate must win more than 50 percent of the vote, as well as more than 25 percent of the vote in two-thirds of the states to avoid a runoff.

If no candidate wins by those margins, a runoff election would take place Feb. 28. If those margins still are not achieved, a third runoff would be held in a week, winnable by a simple majority.

Boko Haram was expected to be a main topic of Kerry's discussions. In a report last week, the Virginia-based Center for Naval Analyses, a federally funded research corporation, called the group a locally focused insurgency largely fueled by bad government.

"The conflict is being sustained by masses of unemployed youth who are susceptible to Boko Haram recruitment, an alienated and frightened northern population that refuses to cooperate with state security forces, and a governance vacuum that has allowed the emergence of militant sanctuaries in the northeast," the report said.

"The conflict is also being perpetuated by the Nigerian government, which has employed a heavy-handed, overwhelmingly (military) approach to dealing with the group and has paid little attention to the underlying contextual realities and root causes of the conflict," the report said. That view comports with the assessment of the U.S. intelligence agencies.

In December, Nigeria canceled the last stage of U.S. training of a Nigerian army battalion, a reflection of strained counterterrorism relations between the two governments.

In April 2014, Boko Haram kidnapped 270 schoolgirls from the northern town of Chibok, prompting international condemnation and a campaign to "Bring Back Our Girls." Most of the girls, however, have not been rescued.

Boko Haram has denounced democracy and is fighting to impose its strict version of Shariah law across Nigeria, whose population of about 170 million is divided almost equally between Muslims in the north and Christians in the south.