Miguel Barreiro, a UN official in charge of the country's disarmament programme, said: "It's the biggest armoury in the country. It contains thousands of tonnes of weapons and ammunition."
The government said the blasts could have been triggered by high temperatures of up to 38C on Thursday.
Unice Mucaphe, the Red Cross programme director for Mozambique, said people were still being evacuated from the affected areas.
"There are eight neighbourhoods where people live which are affected," she said.
"People are still being evacuated, there are houses destroyed. The difficulty is now to locate people because there are lots of children who are lost."
A doctor at the capital's central hospital said a large number of children were among the victims.
"We have had to do many amputations for the children," he said.
Armando Guebuza, the president, called off a visit to South Africa following the disaster.
In a radio address on Thursday, Guebuza appealed for calm.
"Our defence forces are working to assess the situation. We call upon all our citizens to be calm until the situation is rectified," he said.
Maputo's airport, situated near the armoury, remained closed on Friday following the blasts.