Police said they found five bodies of men aged between 25 and 30 after clashes in a rubber plantation in Yala, one of the four southern provinces where more than 2,300 people have been killed in the three-years of fighting.
The separatist insurgency re-emerged in January 2004 in the region, a former Muslim sultanate annexed by Thailand about a century ago.
Four M-16 rifles and combat gear were also found at the site, a police officer involved in the clashes told Reuters news agency.
In another incident, armed men set off a bomb hidden in a motorcycle parked in a busy market in Narathiwat province, killing a Buddhist woman and wounding 11 shoppers.
In Songkhla, a policeman was killed and a pedestrian was wounded by a bomb hidden in a motorcycle parked next to a traffic kiosk, police said.
Other violence, including an arson attack on a petrol station and bombs hidden in rubbish bins, caused some damage but no casualties, police said.
Since last month, security forces have launched almost daily raids on suspected rebels' hideouts in villages and towns and have detained nearly 400 people without charge.
Human rights groups say detainees are exposed to potential abuses by the army, which is given immunity from prosecution under martial law.