Thai police shot dead a gunman who killed three people and wounded three others after a 15-hour standoff, a senior officer said on Thursday.

The shooter started firing in Phetchaburi, about 100 kilometres southwest of Bangkok, at around 3pm on Wednesday, before police surrounded a house he was in.

The standoff ended early Thursday when armed police stormed the building and killed the gunman, who has not been named but was reported by local media to be a 29-year-old former national park official.

"We proceeded step by step, starting with negotiation but he kept fighting back and shot others," Police Lieutenant General Thanawut Wutijarasthamrong said.

"He ran into his room (on the second floor). If we did not have shields, my men would have been shot."

Police found a Glock pistol and two magazines at the scene, but believe the man had more weapons.

Thailand has high rates of gun ownership and there has been a steady number of violent incidents in the past 12 months, including one of the deadliest attacks in recent history: the massacre of 36 people, including 24 children, in northeastern Nong Bua Lam Phu province. (AFP)