Defence chiefs from the US and South Korea on Thursday said they would extend joint military exercises following a string of missile launches by North Korea.
“At this time of heightened tension, our alliance is ironclad,” Mr Austin said.
Mr Austin called the launches “illegal and destabilising”.
The US has 28,500 troops based in South Korea and the allies frequently train together.
The defence chiefs said the coming “Violent Storm” military drills would be extended to include an exercise in which Pyongyang has a nuclear weapon.
Still, the two defence ministers said that they did not feel the US should deploy nuclear devices to the Korean peninsula in response to Pyongyang's latest moves.
Mr Austin condemned the North's “irresponsible and reckless activities” but also hoped for “serious dialogue”.