The youngest son of ex-Bangladesh premier Khaleda Zia left the country on Saturday for Thailand, two days after he was released from detention, police said.

Arafat Rahman Koko, 36, who suffers from lung and heart problems and has severe asthma, was freed on Thursday for two months on the condition that he would not engage in any political or business activity.

"Koko left the country on a Thai Airways flights in the afternoon. He was accompanied by his wife, two daughters and relatives," said airport police chief Ruhul Amin.

His doctor Meshkat Ahmed said Koko, who was brought to Dhaka airport in an ambulance wearing an oxygen mask, would be treated in hospital in Bangkok.

During the visit, he will be required to report to Bangladeshi authorities every three days and is banned from contacting anyone unconnected with his medical treatment, the home ministry said.

Officials said they are also considering releasing Zia, who still leads the Bangladesh Nationalist Party (BNP), and her eldest son Tareque Rahman as part of a deal to make sure BNP participates in the upcoming polls.

Both are also being held on corruption charges.

Koko's release comes a month after another former premier, Awami League leader Sheikh Hasina Wajed, was released and allowed to go abroad after a deal with the government to ensure her party would take part in December polls.

Both the BNP and the Awami League -- the country's two main parties which led governments between 1991 and 2006 -- had said they would not participate in elections unless their leaders were released.

The country's interim leader Fakhruddin Ahmed said earlier this month that his government was ready to extend the "same gesture" to Zia as it has shown to Sheikh Hasina.

But Zia has instead demanded the release of her sons, who she says need medical treatment overseas.