BANGKOK, April 18 (Reuters) - Two Buddhist rubber plantation workers were killed and a senior police officer was critically wounded in suspected insurgent attacks in Thailand's Muslim-majority far south, officials said on Wednesday.

Suspected separatists shot the workers in Narathiwat province late on Tuesday and decapitated one of them, a police officer said in the largely ethnic Malay region, a sultanate until annexed by Thailand a century ago.

"The two were cutting rubber trees when they were hit by a shotgun. One of them had his head cut off and it is still missing," Police Colonel Saravud Netrsawang told Reuters from Narathiwat's Srisakorn District.

One of six bombs planted by suspected Muslim militants in two other districts of Narathiwat on Wednesday seriously wounded a deputy provincial police chief, a hospital official said.

Two of the devices were found and defused in Narathiwat, one of three Muslim-dominated provinces near the Malaysian border where more than 2,000 people have been killed in almost daily separatist violence since early 2004.

But Police Colonel Noppadol Puensomon was hit by a booby trap while inspecting one of the bombs in Takbai District, they said.

"His left leg was blown off and it is impossible to save his almost-severed left arm. He is in very grave condition," a doctor said.

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