Relatives watch the scene of the capsized ferry

Appalling weather conditions have hampered rescue efforts in Bangladesh where more than 500 people are feared dead after a ferry capsized on the Meghna river.

The ferry was carrying about 750 people when it sank on its way from the capital, Dhaka, to southern Bangladesh on Tuesday.

Only four bodies have been recovered so far with more than 500 passengers declared missing.

Strong currents and the depth of the river at the spot of the accident have made work difficult for rescuers seeking to locate the triple-decked ferry, the MV Nasrin-1, which is thought to be lying 60 metres (200 feet) down.

Initial reports suggested the ferry may have been overloaded, not just with passengers, but also with a large cargo of rice and vegetables.

It is understood the vessel had a capacity of just 350.

The accident in treacherous monsoon weather is the fourth sinking involving large loss of life this year, and increases pressure on the government to take effective measures to improve safety, says the BBC's Alastair Lawson in Dhaka.

Bangladeshi Shipping Minister Akbar Hussein said around 200 people had managed to swim ashore, while some others may have been rescued by local fishing boats.

Notorious danger spot

The boat hit turbulent waters near the town of Chandpur just before midnight, as many of its passengers were sleeping.

The spot where the ferry sank - at the confluence of the Padma, Meghna and Dakatia rivers - is notorious for accidents.

Local fishermen and divers were using their boats and fishing nets to desperately search for survivors, officials said.

Hundreds of anxious relatives and survivors lined the shores or joined rescue teams.

"I am holding my breath, still hoping he will be found," a man named Babul, whose five-year-old son is missing, said.

Another anxious mother, Fatima Begum, said: "I will not leave until the ferry is retrieved. My son may be inside it."

One survivor, Samir Chandra Adhikari, said he was on the roof of the vessel and dived overboard when it started to sink, swimming 100 metres to the riverbank.

"The ferry turned aside suddenly. Within moments it started nose-diving and I jumped into the river."

No accurate death toll?

Nearly 220 survivors were taken on to their destinations aboard another ferry after receiving treatment at local hospitals.

Initial investigations suggested the accident happened after one of the ship's two engines stopped, sending it out of control.

It is unlikely that the exact number of passengers on board will ever be known as ferries in Bangladesh rarely have passenger lists or issue tickets.

Another ferry with about 400 people on board capsized in the same spot seven years ago and has never been found.

The authorities have now engaged a Bangladesh Navy ship equipped with echo sounder along with two other rescue vessels to trace the latest sunken ferry.