Police are investigating the roadside killing of three more people in Thailand's Yala province after five policemen were slain in their vehicle earlier in the week.

An unknown number of assailants gunned down two men, aged 52 and 61, as they were leaving their village on a motorcycle in Yala's Bannang Sata district.

Their deaths came on the same day as an assistant village headman was shot in an ambush in Yala's Raman district, the Bangkok Post reported.

Witnesses told police that the man was driving his motorcycle when gunmen hiding in the roadside forest fired on him with assault rifles and handguns. The victim was hit with several bullets in his torso and was declared died at the scene.

Police blame separatist militants in Yala, Thailand's most southerly province, the Post report said.

Earlier this week Thai authorities said they suspect a bomb that exploded killing five anti-drug police officers was the work of drug traffickers. The bomb was planted in a parked pickup truck in the Raman district.

The powerful explosion sent the truck flying from the roadside into a canal. Five of six Thai drug suppression officers in a second vehicle were killed.

Authorities said the truck bomb attack may have been retaliation for recent arrests of drug-trafficking suspects, the Bangkok Post reported.

The explosion was followed by an exchange of gun fire between surviving police officers and the attackers, who then fled, the Post said.

The recent attacks are part of a low-level but deadly rebel movement in the three southern provinces of Narathiwat, Pattani and Yala. The region has a well-established network of insurgents fighting for degrees of regional autonomy and independence.

Government officials in Bangkok and human rights groups have criticized rebel groups for their indiscriminate attacks -- often with roadside bombs or bombs placed in vehicles -- on civilians as well as on the police and security forces.

At the beginning of the month, rebels killed an assistant village chief and wounded six soldiers when three bombs were detonated in different areas of Yala.

The soldiers were wounded with a roadside bomb in one incident and in another incident by a bomb placed in the carrier of a parked bicycle.

More than 5,000 people have been killed and more than 8,400 hurt in the three southernmost provinces and four districts of Songkhla since violence erupted in January 2004, said Deep South Watch, an agency that monitors the conflict.

An Amnesty International report in September called for the rebels to stop their fight against the authorities and civilians -- so-called soft targets.

The report said armed and organized ethnic Malays -- nearly all Muslims -- have been fighting the officially and predominantly Buddhist Thai state.

Amnesty's 64-page report "Unlawful Killings in Thailand's Southern Insurgency" was based on extensive interviews with victims of violence in the four southernmost provinces.

The soft targets include "farmers, house workers, teachers, students, religious leaders, monks, civil servants or persons with vague or tenuous affiliation with the security forces or counterinsurgency efforts."

"Insurgents in southern Thailand are spreading terror among the civilian population by deliberately targeting people with no role in the conflict and no one is immune from attack," Donna Guest, Amnesty International's Asia-Pacific Deputy director, said at the time.

"The insurgents must publicly commit to stopping these unlawful killings immediately."

The report is based on the testimony of 154 interviews with witnesses and survivors, relatives and friends of victims, conducted between October 2010 and July 2011.

The testimonies describe 66 insurgent attacks against civilians in three southern Thai districts: Rangae in Narathiwat province, Yarang in Pattani and Yaha in Yala. Killings also have been noted in parts of neighboring Songkhla province.

"Amnesty International calls on the insurgents to immediately cease attacks deliberately targeting civilians, indiscriminate attacks and other violations of international humanitarian law, many of which constitute war crimes," the report said.