General Joe Dunford, who is the chairman of the Joint Chiefs of Staff, said the Pentagon's role is to bolster diplomatic efforts by providing a clear message about US warfighting capabilities.

"Right now, the military dimension of the problem is reinforcing Secretary (of State Rex) Tillerson's economic and diplomatic pressure campaign," Dunford told reporters travelling with him after he visited US Marines temporarily deployed to an Australian air force base in Darwin.

"His diplomatic efforts are underwritten by a credible military strength."

Dunford stressed the US seeks a peaceful outcome to the crisis over North Korea's push to develop intercontinental ballistic missiles that have the capability of reaching the US mainland.

Still, he said, President Donald Trump has asked him and Defence Secretary Jim Mattis for a "full range" of military options.

Earlier, after having lunch with troops, Dunford responded to a marine's question about how -- in the event of a war with North Korea -- the military would avoid the high casualty rates that were incurred during the Korean War of 1950-53.

"We're obviously a different force than we were even 10 years ago," Dunford said, referencing America's space, cyber and missile-defence capabilities.

"But at the end of the day it will be a nasty war if we fight on the Korean Peninsula and it's going to involve marines and soldiers taking ground, alongside obviously our allies and partners."

"If you are a Marine, and frankly if you are anybody in uniform, if you wake up in the morning always believing that this is the last day that you will be at peace, you are going to be in the right place," added Dunford, who was head of the Marine Corps before becoming chairman of the Joint Chiefs.

"The last thing you want to do when you are in uniform is to be complacent and to think, nah, we are never going to go to war."

The US Marines have been sending troops to Darwin since 2012 on a rotational basis to train with their Australian counterparts and show a regional presence.

Last year, 1,250 were deployed but that number is expected to increase in 2018.