A female suicide bomber killed herself Monday when she exploded a vehicle packed with explosives near a stadium where Nigeria's president had just held an election rally in the northeastern city of Gombe, police said. There were no other casualties.

The explosion occurred in a near-empty area about 1 mile from the venue, which President Goodluck Jonathan had just left, said Deputy Superintendent Fwaje Atajiri.

Two other suicide bombings in Gombe city on Sunday injured a few people but killed only the bombers, he said. A couple sharing a bicycle blew themselves up at a central traffic circle and a man blew himself up at a timber market. All three bombers died, Atajiri said.

No one immediately claimed responsibility. Most suicide bombings are blamed on Boko Haram Islamic extremists who are against democracy and have vowed to disrupt the Feb. 14 elections for president, state governors and legislators in Nigeria, Africa's richest and most populous nation.

The attacks come as the International Criminal Court's prosecutor urged all contestants to refrain from violence before, during and after the vote, with analysts saying the presidential contest is too close to call.

Prosecutor Fatou Bensouda said she would send a team to Nigeria before the election, noting "Experience has shown that electoral competition, when gone astray, can give rise to violence and in the worst-case scenarios, even trigger the commission of mass crimes that shock the conscience of humanity."

Frontrunners are President Goodluck Jonathan, a Christian southerner and Muslim northerner Muhammadu Buhari, a former military dictator. Some 800 people died in protests in mainly Muslim northern Nigeria after Buhari lost the last election to Jonathan in 2011.

Prosecutors at The Hague-based court already are conducting a preliminary probe into alleged war crimes committed by Boko Haram and by Nigerian security forces that could lead to a full-blown investigation.