N’DJAMENA (Reuters) - Four female suicide bombers attacked the Chadian island of Koulfoua on Lake Chad on Saturday, killing at least 15 people and injuring 130 in an attack blamed by security sources on militant Islamist group Boko Haram.

The swampy maze of islands in the border areas between Chad, Niger, Cameroon and Nigeria has become one of the main targets of Boko Haram fighters who can find easy cover in the waterways. The militants are based mostly in Nigeria but have become a wider security threat for the region.

“The provisional death toll is 19 dead, including the four kamikazes, and 130 injured,” said state TV.

A U.N. official said that there was an unspecified number of people displaced by Boko Haram-related violence on the island when the attackers struck around midday on a busy market day.

Two of the blasts rang out from the center of the market and a third in the street as people fled. All of the bombers were female, the security sources added.

Chad extended a state of emergency in the region last month after a double suicide attack that killed 12 people. Dozens of people were killed in multiple bombings in the border town of Baga Sola in October.

But new restrictions, such as bans on motorized canoes aimed at stopping attacks, makes it harder for aid agencies to access the roughly 60,000 displaced people living there.

“Evacuations will have to be done by the army with the assistance of non-governmental organizations and the United Nations to local healthcare centers like Bol,” said Florent Mehaule, head of the U.N. Office for the Coordination of Humanitarian Affairs in Chad.

He said most of the region’s displaced had arrived in April after clashes between Chad and Boko Haram in Niger and Nigeria. Mehaule could not provide exact details of the number of displaced on the island during the attack since they have not been able to reach it for several months.

Lake Chad countries, backed by Benin, have vowed to defeat Boko Haram using forces from an 8,700-strong regional task force which officially become operational in August.

However, security sources say that logistical constraints have caused delays and there are growing signs that national armies are instead acting alone. Cameroon said it conducted a major operation along its long western border with Nigeria, killing 100 militants and freeing 900 people.

Ryan Cummings, chief analyst for Africa at crisis management company red24, said such a strategy would have limited effects.

“Boko Haram requires a simultaneous multilateral counter-insurgency operation which leaves militants with nowhere to run,” he said.