At least 20 people are injured following a bomb blast at a public hospital in the Thai capital on Monday (May 22) morning.
The explosion happened at Phramongkutklao Hospital, which is used by serving and retired members of the military and their families.
But after deputy police chief Srivara Ransibrahmanakul checked the scene in the afternoon, he told Thai media the incident was a bombing.
Thailand's prime minister Prayut Chan-o-cha has called for people not to panic following the attack, which coincides with the third anniversary of the country's 2014 military coup.
"We can confirm at this stage that it is a bomb", he said.
"Right now, authorities are checking out closed circuit cameras", he said.
Three people received serious shrapnel injuries to the face and neck, but most of the victims had minor wounds, said Sansern Kaewkumnerd, a government spokesman.More news: Google Rumored to unveil it's VR Headset in I/O 2017
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It is not clear at this moment if the explosions are related to political conflict, or the long-running separatist insurgency in the Malay-Muslim dominated southern border provinces of Thailand.
Since the coup, the junta, known as the National Council for Peace and Order, has clamped down on dissent and ramped up prosecutions under sedition and royal defamation laws.
But the army chief, Gen. Chalermchai Sittisart, said it appeared that Monday's explosion and two earlier blasts in recent weeks used similar explosive materials and were likely part of an attempt to disrupt the government.
The military has always played a prominent role in Thai life but since the coup it has become embedded in society.
An election is due by the end of next year.
On May 15, a small bomb went off near the National Theater in Bangkok's old quarter, wounding two people.