Authorities in southwest China have confirmed the deaths of 21 workers in two flooded coal mines, state media said Monday, highlighting the dangers of toiling in the nation's collieries.

In Guiyang city, the capital of Guizhou province, 13 people were confirmed dead at the Fuhong mine, following a flood on May 29, the official China Daily newspaper reported.

Only five bodies have been recovered, but experts have determined that the eight miners still inside had no chance of survival.

A preliminary investigation revealed that the mine was not constructed in accordance with an approved design -- a possible cause of the accident.

The second incident, which took place at an illegal mine on May 31 on the border of Guizhou province and the Guangxi region, left eight workers trapped. All of their bodies had been recovered by Saturday, the newspaper reported.

Police have arrested four mine owners, it said.

Last year, 2,433 people died in coal mine accidents in China, according to official statistics, or a rate of more than six workers per day.

Labour rights groups, however, say the actual death toll is likely much higher than official data indicates, partly due to under-reporting of accidents as mine bosses seek to limit their economic losses and avoid punishment.

Fatalities at Chinese coal mines peaked in 2002 when 6,995 deaths were recorded, sparking efforts by the government to boost safety standards.

In its latest campaign, the government last year issued a policy that required six kinds of safety systems, including rescue facilities, to be installed in all coal mines within three years.