The International Campaign to Ban Landmines (ICBL) has called on Bangladesh to release its country representative, whom it says has been detained without charge.
Rafiq Al Islam was arrested at his home in the south-eastern town of Cox's Bazar on August 21. His family told AFP they have received no information about where he is being held.
"We fear for the well-being and even the life of our esteemed colleague," said Liz Bernstein of ICBL in a statement.
"Rafiq has worked openly and co-operatively with Bangladesh officials on the landmine issue for many years, making his arrest as inexplicable as it is appalling," she added.
The regional commanding officer of the police Rapid Action Battalion, Kazi Emadul Haq, said he had received an order from his headquarters to arrest Islam in connection with suspected arms smuggling.
ICBL representative Yeshua Moser Puangsuwan, who has travelled to Bangladesh following Islam's arrest, said Islam's work would have brought him into contact with arms dealers but rejected allegations of arms smuggling.
"We believe that this is an illegal situation," he said.
ICBL is working in Bangladesh to eliminate mines planted by Myanmar security forces in 1993 to prevent Muslim Rohingya refugees fleeing into Bangladesh.
During the 1990s thousands of refugees took shelter in Muslim-majority Bangladesh. They accused the military government of persecution in Myanmar, where most of the people are Buddhist.
Bangladesh also has landmines left from its 1971 war of liberation with Pakistan.