Seoul officials see the talks as the starting point of President Moon Jae-in's initiative to denuclearize and build lasting peace on the peninsula, beginning with a freeze in the North's nuclear program and ending in its complete abolishment.
"I'm announcing today the United States of America will soon unveil the toughest and most aggressive round of economic sanctions on North Korea ever," Vice President Mike Pence said after meeting Japanese Prime Minister Shinzo Abe.
North Korea violated United Nations sanctions to earn nearly $200 million in 2017 from banned commodity exports, according to a confidential report by independent UN monitors, which also accused Pyongyang of supplying weapons to Syria and Myanmar.
South Korea on Tuesday offered talks with North Korea amid a standoff over its weapons programs, a day after North Korean leader Kim Jong Un said he was open to negotiations but that his country would push ahead with "mass producing" nuclear warheads.
Kim Jong-un on Monday warned the United States that he has a "nuclear button" on his desk ready for use if North Korea is threatened, but offered an olive branch to South Korea, saying he was "open to dialogue" with Seoul.
That brings the total number of North Koreans who have defected by taking dangerous routes either directly across the border or by sea to 15 so far this year, including two other soldiers. That is three times the number last year, according to South Korean officials.
US Secretary of State Rex Tillerson on Friday urged North Korea to carry out a "sustained cessation" of weapons testing to allow the two countries to hold talks about Pyongyang's nuclear and missile programs.