Thousands of dead fish floated in a river as oxygen levels dropped following heavy rain in Thailand.

Scores of dead fish surfaced at the Tha Chin River in Nakhon Pathom province, with opportunistic locals visiting to stock up on seafood on May 28.

Footage shows villagers - some arriving from neighbouring provinces - scooping the fish onto their pickup trucks, which were loaded with piles of sheatfish, silver barb, and tinfoil barb.

One of them, Pichitchai Promto, 23, from Kanchanaburi province, said he came to the river because he saw a video online of the fish kill.

Resident Mana Petch-eaw said the phenomenon was rare but sometimes happened after heavy rainfall.

He said: 'The fish may have died due to the lack of oxygen in the water.'

Fish kills may occur after heavy rainfall due to surface runoff introducing a large amount of organic matter into the water.

As microorganisms break down organic material, they may deplete the oxygen supply in the water, causing fish to suffocate.

Thailand and neighbouring countries in Southeast Asia are at the start of their annual monsoon rainy season which sees soaring temperatures followed by powerful rain storms. The rudimentary infrastructure often struggles with the deluge, leading to widespread flash floods.