Story highlights Trump tells journalists Kim not as 'strong' as he thinks

US, South Korean destroyers are holding drills in the Yellow Sea, off the Korean coast

Envoys from US, South Korea and Japan meet in Tokyo

(CNN) US President Donald Trump has said the status quo with nuclear-armed North Korea is unacceptable, calling for new sanctions on the country and criticizing its young leader Kim Jong Un.

"This is a real threat to the world, whether we want to talk about it or not," Trump said Monday at a lunch for ambassadors of countries who sit on the Security Council.

"North Korea's a big world problem, and it's a problem we have to finally solve. People put blindfolds on for decades and now it's time to solve the problem."

Trump's comments come as the USS Michigan, one of US's most powerful submarines, arrived in South Korea in an apparent show of US force and senators have also been summoned to the White House to be briefed Wednesday by top officials about the threat posed by North Korea.

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The US president later told a reception of conservative journalists that Kim wasn't the strong leader he likes to portray himself as.