The Niger government said on Tuesday it had lost 46 soldiers and 28 civilians in a Boko Haram attack on a Lake Chad island at the weekend, the country's heaviest losses yet since it joined a regional offensive against the militants.

Interior Minister Hassoumi Massaoudou said "156 terrorists" were also killed in the assault on an army base on the island of Karamga on Saturday.

The Niger authorities declared three days of national mourning from Wednesday.

Speaking on public radio after visiting the island, Massaoudou said 46 troops had died in the dawn attack, nine were injured and 32 were missing, while "28 island residents were murdered".

He said the Niger army had since retaken control of the island.

It is the first official toll given by Niamey after the battle for the island with the Nigeria-based Islamist group. A Chadian security source on Monday said Niger had lost 48 soldiers and another 36 were missing.

Niger, along with Chad, Cameroon and Nigeria, has recently launched a joint offensive to end Boko Haram's six-year insurgency, which has claimed 13,000 lives and caused about 1.5 million people to flee their homes.

"The village has been completely devastated," the minister said, adding that the country was "horrified" by the heavy toll, one of the biggest setbacks yet in the regional fightback against the Nigeria-based extremists.

Massaoudou said military ground and air operations were under way to track down the missing soldiers and neutralise "the perpetrators of this despicable act whose cruelty is unparallelled".

Niger army chief Seyni Garba said his country's resolve to crush Boko Haram was undeterred by the attack in Lake Chad, where the borders of Chad, Nigeria, Cameroon and Niger converge.

"This act by Boko Haram should not rattle our determination," Garba said on public television, speaking from Niger's southeastern Diffa region just across the border from Boko Haram's stronghold in northeastern Nigeria.

While the Nigerian army has claimed a series of successes against Boko Haram since launching a regional military alliance against the group in February, a string of recent attacks have underlined the continuing risks posed by the extremist fighters.