Story highlights North Korea has been a foreign policy headache for the Trump administration

President Trump and his South Korean counterpart, Moon Jae-in, spoke Sunday

(CNN) President Donald Trump and his South Korean counterpart, Moon Jae-in, have agreed to intensify economic and diplomatic pressure against North Korea's leader Kim Jong-Un -- or "Rocket Man," as Trump nicknamed him -- over his nation's repeated nuclear and missile tests.

The two leaders shared a phone call Sunday and their offices released a transcript of their conversation.

"President Trump and President Moon committed to continuing to take steps to strengthen deterrence and defense capabilities, and to maximize economic and diplomatic pressure on North Korea," the White House said in a statement.

Trump also tweeted about the phone call.

"Asked him how Rocket Man is doing," Trump said on Twitter on Sunday in an apparent reference to Kim. "Long gas lines forming in North Korea. Too bad!"