Nyiragongo volcano close to the city of Goma erupted in the Democratic Republic of the Congo threatening Goma and neighbouring towns.
At least 15 deaths have been confirmed, but the number is likely to rise as officials reach the hardest-hit areas.
More than 170 children are feared to be missing and 150 others had been separated from their families, according to UNICEF, which said centres would be set up to help unaccompanied minors.
The volcano, located 10 km (six miles) from Goma, last erupted in 2002, killing 250 people and making 120,000 homeless.
Rwandan authorities said more than 3,000 people had officially crossed from Goma. Some started to return on Sunday. Others went to higher ground to the west of the city.
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