Development agencies in Bangladesh say at least eight of their workers have been injured in a string of firebomb attacks.
Attacks on NGOs are rare in a country riddled with political violence
Those injured work for the country's two leading non-government organisations (NGOs) providing development assistance to the poor.
NGO officials say they suspect Islamic extremists may be behind the attacks.
Attacks against NGO workers are rare in Bangladesh, despite the violence and political instability in the country.
No-one has claimed responsibility for the attacks which took place on Tuesday and Wednesday.
Two home-made bombs were thrown into the office of the Bangladesh Rural Advancement Committee (Brac) on Tuesday night in Bogra district, about 180 km (110 miles) north of Dhaka.
At least four workers inside the building were hospitalised, police said.
The next evening, three similar bombs were hurled at the offices of the Grameen Bank in district Sirajganj, about 100km (60 miles) northwest of the capital.
Three firebombs were recovered from another Brac office on Wednesday before they could explode, police said.
"As we are working for the empowerment of women and also preach women's liberation, we believe that Islamic groups are responsible for these attacks," the executive director of BRAC, Abdul Muyeed Chowdhury, told BBC.
He said such groups have been opposed their work on various occasions. Mr. Chowdhury is also the president of the NGOs' Federation.
The federation in a statement on Thursday said their workers were feeling insecure and it was becoming difficult for them to continue with their work.
Grameen and BRAC are two of the country's largest NGOs providing development loans to the poor to start small businesses in areas such as handicrafts, poultry and vegetable farming.
Brac also runs charity schools for poor children.
Millions in Bangladesh have benefited from projects funded by these organisations.