"North Korean officers are trained to press their button without any further instructions from the general command if anything happens on their side," Thae told the House Foreign Affairs Committee, referring to a US bombing or military strike.
"We have to remember that tens of millions of South Korean population are living 70 to 80km away from this military demarcation line."
He urged Washington to use "soft power" instead - enforcing sanctions and disseminating information challenging North Korean propaganda to turn people against Kim's authoritarian government.
He also contended that if China allowed an "exit route" for North Korean defectors without fear of forcible repatriation there would be a "massive exodus" to the South that would cause the collapse of the North Korean system.
North Korea has called Thae "human scum" and accused him of embezzling government money and committing other crimes.
Thae's comments come ahead of US President Donald Trump's five-nation trip to Asia that will include a stop in South Korea. The US Administration says it seeks a diplomatic solution to the nuclear standoff but "all options" are on the table.
Trump has threatened the total destruction of North Korea if the US is forced to defend itself or its allies.
A Congressional Research Service report published last week said that conservative estimates anticipate that in the first hours of a conflict, North Korean artillery situated along the frontier could cause tens of thousands of casualties in South Korea, where at least 100,000 and possibly as many as 500,000 Americans live - including nearly 30,000 US troops.
It said a protracted conflict, particularly one in which North Korea uses its nuclear, biological or chemical weapons, could cause enormous casualties on a greater scale.
Some analysts contend the risk of Kim acquiring a nuclear weapon capable of targeting the US homeland is greater than the risks associated with the outbreak of a regional war, the report said.
-AP