By Jonathan Head BBC South East Asia correspondent

Thailand's police force says 23 people have been killed in a tough new campaign against drug dealers.

Four people were shot dead by security forces and 19 were killed in violence between rival gangs, a police spokesman said.

Methamphetamine abuse is a serious problem in Thailand

The new campaign was part of an initiative by Prime Minister Thaksin Shinawatra, who came to office two years ago promising to rid Thailand of the scourge of drugs.

So far he has little to show for his so-called zero tolerance policy, but police appear to be taking seriously his new promise to get even tougher with drug dealers.

Police said that 2,500 people were arrested over the weekend for drug-related offences and that 23 people were shot dead.

Police insisted that most of the deaths were caused by inter-gang violence.

'Shoot to kill'

But human rights groups fear that the authorities are now operating an unofficial and illegal 'shoot to kill' policy towards suspected dealers.

They cite one example where piles of coffins were left outside a police headquarters in northern Thailand as a warning and comments by government officials which appear to condone extra-judicial killings.

The government has already executed dozens of convicted drug dealers since 2001.

But it has been unable to stop the flow of millions of high-addictive methamphetamine pills from Burma which are widely used in Thailand by people of all ages.