A bomb exploded Monday morning inside a military-run hospital in central Bangkok, injuring 21 people including soldiers, Thai police said.

 thaiblast (Credit: Thai police)

(Image provided by Thai police)

A bomb exploded around 10 a.m. local time in the business affairs section of the Phra Mongkutklao hospital where medical bills are paid. A guest room for very important patients is located nearby.

A gas explosion was first suspected of causing the explosion, and 24 people were reported to have been injured. But Deputy Police Chief Srivara Ransibrahmanakul later said a bomb had exploded, and number of injured was revised to 21, mostly from cuts caused by broken glass.

Lt. Gen. Saroj Kaokajee, commander of the Army Medical Department, said the injured included several mid-ranking soldiers, but no one was in serious condition. He said the explosive device was low-powered and thus did not cause severe damage.

The Royal Thai Army and police immediately launched investigations into who is responsible for the bombing, which occurred on the third anniversary of seizure of power by the Thai military from the previous government of the Pheu Thai Party.

Government spokesman Sansern Kaewkamnerd urged the public to remain calm and said Prime Minister Prayut Chan-O-cha had ordered proper care for those injured.

Regarding the motivation for the bomb attack, both the police and Thai military said it was too soon to know, adding it was also unclear whether Monday's explosion is connected to last week's bomb blast in front of the National Theatre.