U.S. imposes more North Korea sanctions, Trump warns of 'phase two'

U.S. Treasury Secretary Steven Mnuchin announces on Friday what he said was the largest North Korea-related sanctions in a bid to disrupt North Korean shipping and trading companies and vessels and to further isolate Pyongyang, in the press room at the White House in Washington, U.S. February 23, 2018. REUTERS/Jonathan Ernst

WASHINGTON/SEOUL (Reuters) - The United States said on Friday it was imposing its largest package of sanctions to pressure North Korea to give up its nuclear and missile programs, and President Donald Trump warned of a "phase two" that could be "very, very unfortunate for the world" if the steps did not work.

In addressing the Trump administration's 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.