BANGKOK, May 2 (Reuters) - Two Muslims and a Buddhist have been killed in southern Thailand in two separate incidents bearing the hallmarks of reprisal attacks in the violence-plagued, predominantly Muslim region.

Four suspected Muslim militants armed with automatic rifles attacked the district office of a Buddhist volunteer militia in Pattani province late on Thursday, killing one Buddhist and wounding another, police said.

Less than an hour later, a hand grenade was thrown into a small mosque in a nearby district where about a dozen Muslims were praying. Two people were killed and eight wounded, police said.

Army spokesman Acra Tiproche said it was too early to talk about reprisals between religious groups in the region, where more than 3,000 people have died in a four-year Malay Muslim separatist campaign.

"I can't say at this point whether they are connected or not," Acra said from Bangkok.

On Wednesday, a roadside bomb exploded beneath a police vehicle in Pattani, killing three officers.

The provinces of Pattani, Yala and Narathiwat, abutting predominantly Muslim Malaysia to the south, were an independent Muslim sultanate until their annexation by Buddhist Bangkok a century ago.

No group has claimed responsibility for any of the almost daily bombings, shooting and arson attacks which have ravaged the local economy since January 2004 and driven out many in the 20 percent Buddhist minority. (Reporting by Surapan Boonthanom; Writing by Vithoon Amorn; Editing by Ed Cropley and Valerie Lee)

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