The military is on high alert after increasingly brutal and sophisticated attacks by southern insurgents claimed a total of 21 lives, both soldiers and civilians, in a single day on Thursday, Army spokesman Acra Tiproch said yesterday.

Militants bombed and killed 12 rangers in Yala's Bannang Sata district on Thursday night. The attack was similar to one on May 9 in which seven soldiers were killed.

"Again this is a great loss for the Army. The insurgents have adopted similar tactics to those they used in Narathiwat. They bombed the soldiers and brutally killed the injured to make sure that no one survived," the spokesman said.

Eleven Army rangers died at the scene after militants set off a remote-controlled roadside bomb as they were travelling in a pickup along a back road linking Yala and Betong shortly before 10pm. They were returning to their camp from crowd-control duty at a protest by local residents. The 12th soldier died in a hospital.

"This was the biggest single attack yet," the spokesman said.

Acra said that Army investigation into the attack showed that the insurgents had killed the injured soldiers by either shooting them at point-blank range or strangling them to make sure that all were dead.

"The militants have stepped up attacks in order to increase pressure on authorities," Acra said.

He said the rebels were hoping to provoke a heavy-handed reaction from security forces, who could then be blamed for committing atrocities against residents in the deep South.

Acra also referred to a separate attack in Songkhla's Saba Yoi district on Thursday night when the insurgents sprayed bullets at villagers shopping in an evening market, killing five.

He said that the culprits behind the attack were believed to be same groups operating in Yala's Yaha district. They created violence in Saba Yoi district to draw police and soldiers from Yala.

Four other people were killed in scattered attacks in the region.

Acra described the single-day death toll of 21 as "shocking".

Meanwhile, Prime Minister Surayud Chulanont vowed not to retaliate.

"We must be patient ... we have to be firm on the rule of law. We will not do anything to cause more problems and injustice," he said.

However, he said it would be impossible to win the support of many militant sympathisers.

Meanwhile, about 2,000 students and villagers protested at a mosque in Pattani province for the second day yesterday to demand an end to the state of emergency declared in the region and an immediate withdrawal of troops.

The core protesters were Muslim students from Ramkamhaeng University in Bangkok. They announced through a speaker that they represented the university's student organisation and hid their faces with scarves. They seemed to take control of the mosque as they guarded all entrances and screened people who wanted to enter.

The president of Ramkamhaeng University Students Organisation, Sikanan Nulek, said that while it was true that the protesting students were members of the body, they did not represent the organisation.

"The students who are now protesting in Pattani are members of the university students organisation. But they are acting on their own. Before they went down to Pattani they asked me to join them, but I refused," he said.

They said they wanted to bring justice to local people who were under attack from soldiers and the authorities, he added.