The project, MASAM, said on its website that the six experts, whose nationality was not given, died in a blast at a depot containing mines and other explosive devices which were to be destroyed.
In January, five foreign experts also with MASAM - two from South Africa, a Croatian, a Bosnian and a Kosovar - died in an accidental explosion in the central province of Marib.
Five killed in heavy fighting in Yemen's Taez: MSF
Rights groups say landmines have killed and injured hundreds of civilians and blocked aid deliveries since a Saudi-led coalition intervened in the government's war against Huthi rebels in 2015.
According to the World Health Organisation, nearly 10,000 people - most of them civilians - have been killed and more than 60,000 wounded in fighting since March 2015.
Rights groups say the toll could be much higher.
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