In early October, Azarov said there were 23 million people living in Ukraine.
"According to the most modest calculations, only 19 million continue to live in the territory controlled by the regime in Kiev. It's a scary figure," he wrote on social media.
He attributed the population decline to a massive outflow of workforce, in addition to combat casualties.
"Imagine that peace will be restored tomorrow. And a border of some sort will be drawn. Ukraine will need to be rebuilt! Who will do it if there is now a gaping workforce deficit?" Azarov added.
The former minister complained about a breakdown in services in Ukraine. He said Ukraine would need highly skilled workers, such as engineers and technologists, to rebound from the conflict.