The Thai authorities are on alert after an explosion at a Bangkok military hospital wounded 21 people, one of them seriously.

The blast at Phramongkutklao Hospital, which took place yesterday morning in a waiting room for military officers, appeared to be caused by an improvised explosive device. By afternoon, only eight of the victims remained warded. One needed surgery to remove shrapnel from her face.

Images carried by Thai media showed covers of seats in the waiting room ripped by the blast, possibly from the nails that were recovered from the scene.

"It was intended to take lives because (the bomb) was loaded with lots of nails," army chief Chalermchai Sitthisart said. "This incident was meant to cause instability and chaos."

The blast comes on the third anniversary of the 2014 military coup which overthrew a caretaker government led by the Puea Thai party.

The kingdom remains under military rule as new laws are drafted to prepare for fresh elections, expected some time next year.

Soldiers swarmed the hospital in Ratchawithi Road immediately after the blast, cordoning the affected section off from reporters as bomb experts scoured the scene. Other parts of the hospital continued receiving patients.

Deputy national police chief Srivara Ransibrahmanakul declined to speculate on the motive but told the media that traces of a battery and electrical wires were also found at the scene.

"It happened at the hospital, think about it," he said. "Even in the three southern (border) provinces, we have never had this."

Separatists have been waging an insurgency in the Malay-Muslim dominated provinces of Yala, Pattani, Narathiwat and a part of Songkhla, which has claimed more than 6,000 lives since 2004.

Most of the militants' bombings and ambushes of soldiers and policemen have been confined to that region.

Meanwhile, Thai police are still investigating two separate explosions that took place in the capital recently.

On May 15, a small blast occurred outside Bangkok's National Theatre in the heart of its historic quarter. A senior policeman initially denied it was caused by a bomb, but later reversed his position.

On April 5, another explosion went off in Ratchadamnoen Avenue, also in the historic quarter, injuring two street cleaners nearby.

Sporadic bombings have occurred in Bangkok over the past decade of political turbulence leading up to the 2014 coup, but the deadliest in recent times took place in 2015 at the Erawan Shrine, killing 20 people.

Yesterday, unlike in the previous two years, there were no visible street protests in Bangkok to mark the third anniversary of the coup.

•Additional reporting by Yasmin Lee Arpon in Bangkok