(Reuters) - Boko Haram militants have seized a strategic island in Lake Chad from Niger's army, killing "many" soldiers in the first attack in Niger for weeks, army sources said on Sunday.

Hundreds of armed fighters from the Islamic militant group aboard motorized canoes attacked the island of Karamga at dawn on Saturday, army and government sources said.

"There were many (Niger soldiers) dead on Saturday, considerably more than in the first attack," said one of the army sources, referring to a battle in February for control of the same island in which seven Niger soldiers died.

A second army source confirmed that Niger soldiers had been killed in the fighting, adding that a counter-attack to clear the island of militants was still ongoing.

(Reporting by Abdoulaye Massalaki; Writing by Emma Farge; Editing by Gareth Jones)