Earlier, witnesses had spoken of about 50 dead bodies after the 2am (local time) blast in an industrial district of the country's commercial hub.
But a police officer said 12 men and five women had been confirmed dead as of 6 a.m. He said further deaths were expected.
Some witnesses said a fire had broken out at a chemical storage warehouse, causing an explosion that triggered several smaller blasts. Police said three firemen were among the dead.
Fire engines from all parts of Yangon were at the scene, witnesses said. Fire was reported to have spread to a nearby shipyard and factories.
Television pictures showed rescue teams carrying casualties on stretchers in the middle of the night around what appeared to be badly damaged buildings.
The shock from the loud explosion in the area was felt by many people in eastern and central Yangon.
Police would not comment on the cause of the disaster.
"There are many casualties. We are not in a position to give you further information. We are still looking into it," a second police officer from Mingalar Taungnyunt Township Police Station told Reuters.
Last Wednesday, a woman was killed and another injured when what was described as a bomb exploded in a public toilet near the main campus of Yangon University.
Bomb blasts are common in Myanmar, which has been torn by ethnic rebellions and armed struggle against successive governments since independence in 1948.
