Three small explosions Tuesday at separate train stations across Bangladesh left one bystander injured in an attack claimed by a group opposed to non-government development agencies, a railway spokesman said.

The explosive devices had been hidden in small bags at train stations in the capital, Dhaka, and the cities of Sylhet and Chittagong, railway spokesman Shafiqul Alam Khan said. One rickshaw puller was slightly injured in Chittagong, and police were investigating the attacks.

Metal plates found found near the bomb sites were etched with messages from a shadowy group warning people working for non-governmental development agencies in Bangladesh they would be killed unless they quit their jobs by May 10, Khan said.

Many Islamic groups in Muslim-majority Bangladesh oppose the activities of such agencies, which typically are funded by Western donors and employ women.

"Stop associating with nonbelievers. Stop working for NGOs by May 10. Or prepare for death," said the etched messages signed by Jadid Al Qaeda Bangladesh, a hitherto unknown group.

The messages also warned people in Bangladesh against associating with Kadianis, a minority Islamic sect whose members are deemed nonbelievers by many mainstream Muslim groups.

The explosions took place early Tuesday near a ticket counter in Dhaka, under a seat at a waiting lounge in the northwestern city of Sylhet, and on a sidewalk outside the station in southeastern Chittagong.

Tuesday was May Day, a public holiday in Bangladesh.

There were no reports of casualties or damage in Dhaka or Sylhet.

"There was a loud sound," said Khan, who was at Dhaka's Kamalapur station during the blast there. "Bomb experts are investigating the type of explosives."

In recent years, Bangladesh has been wracked by a violent campaign by militants wanting to establish strict Islamic rule in the country, now governed by secular laws.

In 2005, a series of deadly bombings blamed on the Jumatul Mujahedeen Bangladesh group killed at least 26 people and wounded dozens more across the nation.

The banned group's top operatives were convicted and recently hanged.