Somali Islamist group al-Shabab has told the BBC it is holding the bodies of 18 Burundian peacekeepers following heaving fighting in the capital.
The Burundian government had confirmed the death of six of its soldiers but there is growing suspicion that the number killed was much higher.
The AU troops, along with those from the interim government, have captured ground from the militants recently.
But the AU mission has been reluctant to give casualty figures.
A United Nations source said 30 Burundians had been killed plus 10 Ugandans and dozens of peacekeepers had been injured.
Two Nairobi-based diplomats, requesting anonymity, told AP news agency that the number of casualties was even higher and that at least 43 Burundian and 10 Ugandan troops had been killed since 18 February.
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More News On Somalia's Civil War
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