According to sources, the warring parties entered the city, which has around one million inhabitants, and met virtually no resistance. Earlier in the day, they are said to have taken control of the airport in Bukavu.
Save the Children reports that nearly 700,000 of the population in Bukavu are children.
A few weeks ago, M23 captured the city of Goma and have since advanced southwards. The new offensive has resulted in thousands of people being killed and raped. Children have been subjected to many of the rapes, according to Unicef.
"In the provinces of North and South Kivu, we are receiving horrific reports of serious abuses against children by parties to the conflict, including rapes and other forms of sexual violence on levels that exceed anything we have seen in recent years," says Unicef chief Catherine Russell in a statement.
According to the UN, nearly 3,000 people have been killed and hundreds of thousands displaced in eastern Congo since the turn of the year.
