A land rights campaigner was gunned down outside his home in southern Thailand, police said Friday, in what the UN said is the fourth killing linked to land activism in the region in under a year.

Chai Bunthonglek, a village leader from the Khlong Sai Pattana community in the southern Surat Thani province, was shot dead by two gunmen who arrived on a motorcycle on Wednesday evening, police said.

"He was shot three times and died instantly at the scene," Lieutenant Chawalit Thongma at the Chaiburi district police station told AFP, adding that investigators were focusing on his activism as the main motive for his killing.

The UN's regional human rights body called on police to swiftly find Chai's killers, adding that the murder of activists in southern Thailand had become all too common.

"Alarmingly, Chai is also the fourth human rights defender who works on land rights in the south of Thailand that has been killed since May 2014," the United Nations Human Rights Office for South East Asia (OHCHR) said in a statement.

The OHCHR said the Khlong Sai Pattana community had been involved in a long running dispute with a large palm oil plantation over access to land.

Three other members of the community have been killed since 2010.

"Despite the police investigation, no one has been brought to justice for these murders," the statement read.

Enforced disappearances and extrajudicial killings of activists in Thailand are commonplace, with few killers facing justice.

According to the Asian Federation Against Involuntary Disappearances there are at least 81 open cases of enforced disappearance in Thailand dating back as far the mid-1990s.

One case that has largely flown under the international radar is that of Por Cha Lee Rakcharoen, a Karen rights activist also known as Billy, who was apprehended by national park officials in Thailand on April 17, ostensibly for illegal honey gathering.

He was en route to meet fellow ethnic minority Karen villagers to help them file a lawsuit accusing authorities of torching the homes of 20 families in Kaeng Krachan National Park in 2011. (+++++)