(Reuters) -At least four people died after a boat carrying 20 people sank in Bangladesh's Buriganga river near the capital Dhaka on Sunday, a fire service official said. 

Most of the passengers were believed to have swum ashore as the water bus sank close to the bank, according to fire service official Anwarul Islam.

Four bodies have been recovered and the rescue operation is ongoing, police official Sahabuddin Kabir told Reuters.

Seven people have been rescued and sent to hospitals, police said.

Hundreds of onlookers gathered at the site as rescuers in multiple boats scanned the river with flashlights, according to a Reuters witness.

(Reporting by Ruma Paul in Dhaka; Writing by Shubhendu Deshmukh;Editing by Andrew Cawthorne and Emelia Sithole-Matarise)