The Chadian Army has launched a "major operation" to flush out Boko Haram jihadists from islands in Lake Chad, sparking violent clashes between soldiers and the Islamic State-affiliated group.

Already, Nigerian President Muhammadu Buhari is in neighbouring Cameroon for talks on combating the regional threat posed by the extremist group, whose six-year insurgency has claimed 13,000 lives and caused about 1.5 million people to flee their homes.

On Monday, "violent clashes" raged near Baga Sola, one of the main towns in the shallow, marshy waters of Lake Chad, a Chadian security source told AFP.

He added that Chadian forces had earlier "intercepted fleeing Boko Haram elements" around 20 kilometres (12 miles) to the southeast.

The four countries bordering the lake -- Chad, Cameroon, Nigeria and Niger -- set up a military coalition against Boko Haram early this year headquartered in the Chadian capital N'Djamena.

Chadian forces have "around 1,000 men positioned to occupy all the islands and neutralise Boko Haram", the security official said.

The jihadists, now affiliated with the Islamic State group, have been using the islands as a rear base after being routed from their traditional strongholds in Nigeria by the four-country offensive.

The network of hundreds of islets separated by channels hidden by tall grass provides cover for the militants to steal livestock and crops from local inhabitants.

According to the security official, the Lake Chad clashes erupted Saturday on Midi Koutou island, leaving six Boko Haram fighters dead and 15 wounded. A Chadian soldier was also said to have been killed.

The source said Boko Haram "kidnapped many women and children" as they fled Saturday's fighting.

A source close to local authorities said that, following a request from the central Chadian government about two weeks ago, an operation was launched "to evacuate the islands" on the Chadian side of the lake and that about 90 percent of residents had been moved to the mainland since.

"The army forcibly entered almost all the (Chadian) islands," the source said, adding the militants were dispersed across "mainly little islets and villages".

Buhari, meanwhile, will travel to Cameroon on Wednesday for talks with his counterpart Paul Biya on the Boko Haram insurgency, Nigerian presidential spokesman Femi Adesina said.

A new wave of violence has claimed 800 lives since the Nigerian president came to power in May vowing to crush the insurgency.

Recent days have seen a spate of Boko Haram raids and suicide bombings in the region, with seven people killed in Cameroon at the weekend, three of whom were beheaded.

Boko Haram earlier this month claimed responsibility for twin suicide bombings in N'Djamena that left 38 people dead, the SITE Intelligence Group reported.

Three days later, at least 15 people were killed in a suicide bomb attack at a crowded market in the Chadian capital.

The European Union on Monday expressed solidarity with the anti-Boko Haram campaign.

"Cameroon, Chad, Niger and Nigeria are sparing no effort in fighting terrorism in the region... The European Union supports the efforts for comprehensive bilateral and regional cooperation to tackle these challenges," it said in a statement. (AFP)

*Photo shows Boko Haram militants

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