Somalia's hardline Islamist Shebab rebels on Monday dismissed a key donor conference due to open in Brussels as "Belgian waffle" and a waste of time.

Shebab said in a statement on Twitter that "the billions promised will most likely be unpaid, the paltry sum given to the apostates," its term for the Somali government, "will be lost in corruption".

"It's a bit like Belgian Waffles: sweet on the outside but really has not much substance to it. They are just hollow promises of Kufr", or infidels, the group said.

Some 50 high-level delegations from Africa, Europe, the Gulf and elsewhere are expected for the "New Deal in Somalia" event, bringing together President Hassan Sheikh Mohamud, other Somali leaders, world aid groups and global finance institutions.

But Shebab, who are fighting to overthrow the internationally-backed government, said that "conferences never really had any meaningful impact on the ground here in Somalia and this one will be no different".

"It's a waste of effort," the group said.