KANO, Nigeria (AFP) - Three girls staged suicide bombings in the Nigerian city of Damaturu on Friday, killing at least 13 people as residents prepared for the end of Ramadan, police said.

The attacks, in a northeastern area hard hit by the Boko Haram insurgency, came just days before Nigeria's new President Muhammadu Buhari travels to Washington for talks with US counterpart Barack Obama.

The bombings came on the day the country's new army chief was due to visit the city to meet soldiers battling Boko Haram.

Buhari is expected to use Monday's meeting with Obama to push for US help to tackle the violence, which has surged since he took office in May, claiming more than 700 lives.

Boko Haram have increasingly used young women and girls as human bombs over the past year as part of campaign of terror, which has left 15,000 people dead and 1.5 million homeless since 2009.

In a sign of how the violence is spreading across the region, a soldier and 19 Boko Haram members were killed in a shootout following an insurgent raid on an army post at Lake Chad, a security source in neighbouring Chad said.

Chadian President Idriss Deby Itno said he would not back down against the Islamists, vowing: "Boko Haram will disappear one day."

In the Nigerian attacks, residents said twin explosions near a prayer ground in Damaturu killed two people, before a third blast moments later near a mosque that left another 11 dead.

Markus Danladi, Yobe state police commissioner, confirmed 13 people were killed.

"The attacks were carried out by three underage girls. Fifteen people were also injured in the attacks."

Army spokesman Sani Usman however gave a toll of nine dead and 18 wounded, adding: "Troops and security agencies responded immediately to the incidents. The situation is currently under control."

Deadly Ramadan

The attacks followed a double bombing at a market in the town of Gombe, south of Damaturu, on Thursday that killed at least 49 people who were shopping for the Eid celebrations.

"There were two blasts near the Eid prayer ground," said Ahmad Adamu, a security volunteer in Damaturu said of Friday's attacks.

"The first blast went off around 7.15 am (0615 GMT) while security volunteers... were waiting for the worshippers so they could assist in crowd control," he said.

"While we were attending to the victims, we heard another blast about 500 metres (yards) away."

This year's Ramadan has been particularly deadly in Nigeria, with suicide bombers hitting mosques and worshippers gunned down as they prayed.

UN Secretary-General Ban Ki Moon through his spokesman condemned the attacks at "a sacred time for families and communities to come together," saying they "constitute an assault on the beliefs of all people".