In normal times, the two towns have a combined population of more than 150,000. But aid workers said they have no idea how many people are there now.
At least 250,000 people have been displaced by 10 weeks of fighting between army troops and rebels led by Nkunda.
A rebel spokesman said any aid workers who wanted to help civilians trapped on rebel-held territory would be safe.
DR Congo’s armed forces are notoriously ill-disciplined soldiers, historically better at looting than standing their ground. In recent days, some have been seen manning checkpoints drunk.
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