N'DJAMENA (Reuters) - A landmine planted by Islamist group Boko Haram killed four Chadian soldiers on patrol near Chad's border with Niger on Saturday, two security sources said.

They were traveling in a vehicle that rode over the mine at Kaiga Kindji, in the Lake Chad region, which has been plagued by the militants since 2009.

The Nigerian-based Boko Haram wants to create a breakaway Islamic state in the region and once occupied an area the size of Belgium.

But a regional offensive led by Nigeria, Chad, Cameroon and Niger - the four countries most affected by Boko Haram - has chased it out of much of that territory.

The group has in response retreated to Nigeria's Sambisa forest, from where it has fought a guerrilla campaign against civilians and security forces.

Boko Haram is thought to have killed as many as 15,000 people since the launch of its insurgency seven years ago.

Its insurrection has strangled economic and farming activity around Lake Chad, leaving tens of thousands hungry.

Nearly half a million children around the lake face "severe acute malnutrition" due to drought and the insurgency by Boko Haram, UNICEF said on Thursday.

(Reporting by Madjiasra Nako; Writing by Tim Cocks; Editing by Andrew Roche)