A bomb exploded at an army-run hospital in Bangkok on Monday, wounding more than 20 people, Thai authorities said.

Investigators confirmed the blast was caused by a bomb after they found remnants of batteries and wires on the ground floor of Phramongkutklao Hospital, said Srivara Ransibrahmanakul, the deputy national police chief.

"We can confirm at this stage that it is a bomb," Ransibrahmanakul said.

At least 21 people were wounded, one of them severely, said Lt. Gen. Saroj Kiewkajee, a hospital official. Thirteen people were discharged soon after the explosion.

Though the explosion came on the anniversary of the May 22, 2014 military coup that overthrew a democratically elected government, authorities are not connecting the two incidents at this time.

"Please don't connect this with any other incidents because they may or may not be connected,"said Sansern Kaewkumnerd, a government spokesman. "We have to wait for officials to investigate details from surveillance cameras in the area."

It's unclear who is responsible for the attack at the hospital, which is run by the military but is also open to civilians.

Since the 2014 coup, at least six explosions have occurred in Bangkok. Last Monday, a bomb went off in front of the country's National Theater. Last month, a similar explosion took place in front of an old government lottery office.

Most of the bombs in Bangkok caused only minor damage except for a blast on August 17, 2015, that killed about 20 people near a popular Hindu shrine.

The Associated Press contributed to this report.