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.
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 Organization, 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.