DHAKA (AFP) - A respected Bangladeshi professor who vanished more than a month ago has returned home "mentally traumatised", his family said Friday (Dec 22), amid a spate of disappearances in recent months.

Mubashar Hasan, an expert on Islamic militancy at North South University in Dhaka, had not been seen since November 7, with an official search unable to locate the associate professor.

His disappearance was the ninth high-profile case since July in Bangladesh, where the government has been accused of abductions and unofficial detention of critics and opponents in secret jails.

"He was physically okay but mentally traumatised," Mr Hasan's sister Tamanna Tasmin said, after her brother returned home late Thursday evening. "We are extremely happy that my brother has returned."

The professor told local broadcasters that he had been "abducted for ransom", but did not elaborate on his whereabouts for the past six weeks.

Local police chief Jahangir Kabir Khan said Mr Hasan told investigators he had been taken by "strangers".

In recent months opposition political figures, a businessman and journalists have vanished in Bangladesh.

This week a prominent journalist reappeared more than two months after going missing in similarly mysteriously circumstances. A retired diplomat who went missing earlier this month has still not been found, his family says.

Rights groups urged authorities to bring those behind the spate of abductions to heel.

"These disappearances should stop immediately. It is an international crime," Adilur Rahman Khan, from Bangladeshi rights watchdog Odhikar, told AFP.

The government of Prime Minister Sheikh Hasina has been accused of illegally detaining hundreds of people, sometimes for months on end.

Human Rights Watch in July said at least 90 people were victims of forced disappearances in 2016, including nearly two dozen who turned up dead.