If you weren't paying attention, you might have missed it; but during today's joint press conference with Aussie PM Turnbull, US President Trump let slip a brief comment that the rest of the world should likely be paying close attention to.

After unveiling the "heaviest sanctions ever imposed on a country before" against North Korea earlier in the day, President Trump told the gathered media that the US will go to "Phase 2" if those sanctions do not have the desired effects of denuclearizing the Korean peninsula.

As Reuters reports, in addressing what the Trump administration calls its biggest national security challenge, the U.S. Treasury sanctioned one person, 27 companies and 28 ships, according to a statement on the U.S. Treasury Department’s website.

The United States also proposed a list of entities to be blacklisted under separate United Nations sanctions, a move “aimed at shutting down North Korea’s illicit maritime smuggling activities to obtain oil and sell coal.”

The U.S. Treasury said the sanctions were designed to disrupt North Korean shipping and trading companies and vessels and further isolate Pyongyang, but as we noted previously Russian and Chinese ships have been "caught red handed" breaking the sanctions.

All of which led to his comments during today's press conference during which Trump made apparent reference to military options his administration has repeatedly said remain on the table.

"If the sanctions don’t work, we’ll have to go phase two," Trump said. "Phase two may be a very rough thing, may be very, very unfortunate for the world. But hopefully the sanctions will work." The president did not specify exactly what he meant by 'Phase 2' and qualified the statement saying that he didn’t think he was "going to exactly play that card."

As a reminder, in August, Trump threatened to go beyond sanctions by bringing “fire and fury like the world has never seen,” although his administration has repeatedly said it prefers a diplomatic solution to the crisis.

After today's comments, the big question on everyone's mind is - what is "phase 2"?