At least 23 people, the majority of whom are children, are dead after a boat sank in the River Nile in Sudan.

The boat was carrying more than 40 pupils, on their way to school in the north of the country, before suffering a technical failure, according to Sudan's state news agency.

At least 22 students and a woman were killed in the country's northern River Nile state, with civil defence forces searching waters for the missing, but yet to recover any bodies.

A female hospital employee was among those drowned some 470 miles north of Sudan's capital Khartoum.

"The accident was caused by engine failure halfway across because of a strong current," said the SUNA news agency.

It also reported the small boat was overloaded with about 30 sacks of sweet potatoes and 10 bags of grain, in addition to the human cargo.

One witness said at least nine children survived the tragedy.

The River Nile state is one of 18 wilayat (states) of Sudan, with an estimated population of more than one million, where locals rely on wooden boats to cross the waterway.

Sudan experiences monsoon rains between June and November, which cause the River Nile and its tributaries to overflow.

United Nations aid agencies regularly warn of floods during this period.

At nearly 6,700km in length, the River Nile is argued by many to be the world's longest river.

It is an important transport route for goods and people in much of northeastern Africa.

In August 2000 50 students drowned when their wooden barge overturned southeast of Khartoum.

Another boat tragedy, in September 2014, saw 13 people drowned north of the capital.