Thousands of refugees have fled to Chad from northern Nigeria

Attack is suspected to be linked to Nigerian-based Boko Haram, who have expanded attacks into Cameroon, Chad and Niger

Three female suicide bombers have attacked a busy market on an island in Lake Chad leaving at least 27 people dead and 90 injured.

Nigeria-based Islamic terrorist group Boko Haram, who pledged allegiance to ISIS earlier this year is suspected of carrying out the attack, according to Chad's security forces.

Tens of thousands of refugees have fled Nigeria to Chad, which was branded a 'war zone' by an UNHCR spokesman in October.

At least 27 people have been killed by three female suicide bombers, suspected to be tied to Boko Haram, at a market in Lake Chad. Chad has been repeatedly targeted by Boko Haram militants, as it is a major force in helping Nigeria fight the group

Exact details of the number of displaced on the island during the attack are unclear as aid agencies have been unable to reach it for several months.

The swampy maze of islands in the border areas between Chad, Niger, Cameroon and Nigeria has become one of the main targets by Islamist militants from Boko Haram who can find easy cover in the waterways.

And Chad has been repeatedly targeted by Boko Haram militants, as it is a major force in helping Nigeria fight the group.

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

It is currently hosting a new regional force set up to tackle the Nigeria-based militant Islamists.

The country has welcomed tens of thousands of refugees displaced from their homes by the conflict with Boko Haram according to UNHCR.

Lake Chad, which borders Niger, Cameroon and Nigeria has been a repeated target for Boko Haram. Aid agencies have been unable to access the region for months

But new restrictions, such as bans on motorised 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 organisations and the United Nations to local healthcare centres 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.

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 simultanous multilateral counter-insurgency operation which leaves militants with nowhere to run,' he said.

Two suicide bombings also by women killed at least three people in November in Ngouboua village near Lake Chad.

Five coordinated suicide bombings in October killed at least 36 people and wounded some 50 others in the western village of Baga Sola near Lake Chad that is home to thousands of Nigerians who have fled the extremists' violence.

Boko Haram's 6-year uprising has killed some 20,000 people. Above, aftermath of a twin suicide attack in in Kano, Nigerian which killed 15 people on November 18th

Boko Haram's 6-year uprising has killed some 20,000 people.

The Nigeria militants have this year expanded attacks into Cameroon, Chad and Niger, countries contributing troops to a regional force formed to wipe out the extremists.

Forces from Nigeria and neighboring Chad earlier this year drove the extremists out of cities and towns in northeaster Nigeria where they had proclaimed an Islamic caliphate.

Troops from Cameroon, Nigeria, Chad and Benin had last week launched operations in the Lake Chad region and the Sambisa Forest, arresting 100 militants, killing 100 others and freeing 900 hostages, according to Cameroon's government.

The bombings in Chad come as Nigeria's intelligence agency says it has arrested nine alleged Boko Haram extremists plotting attacks on Abuja, the capital, over the festive season.

Saturday's statement follows a warning Friday from the U.S. Embassy that extremists may be planning attacks on hotels favored by Westerners.

One of the nine men arrested was carrying out surveillance of a 'high-profile hotel,' said Nigeria's Department of State Security. It said all nine were detained in the past month and had infiltrated Abuja, the capital in central Nigeria, from the country's northeastern area where most extremist attacks occur.

Two female suicide bombers, one thought to be as young as 11, blew themselves up at a busy market in the Nigerian city of Kano on November 18th.

Chad hosts some 438,000 refugees, including 350,000 Sudanese, 90,000 from Central African Republic and some 13,000 Nigerians.