President Barack Obama is in South Korea Friday on the second stop of his four-country Asian tour.
Obama's visit comes during a time of mourning for the Asian nation.
Rescue crews are still struggling to recover bodies from last week's sunken ferry, with more than 300 dead or still missing.
Officials investigating the cause of the disaster are looking at what cargo the ferry was carrying in addition to its passengers and crew.
A naval expert says 3,600 tons of cargo or more could have caused the ferry to tip if it turned too sharply.
Obama and South Korean President Park Geun-Hye observed a moment of prayer for the victims of the tragedy.
Meanwhile, there's some concern that North Korea may carry out a fourth nuclear test during the president's visit to show its prowess in the region. North Korea carried out nuclear tests in 2006, 2009, and 2013.
Before arriving in Seoul, Obama warned Pyongyang of a "firm response" should it make "the mistake" of carrying out another test, Britain's The Guardian reported.
"North Korea has engaged in provocative actions for the last several decades. It's been an irresponsible actor on the international stage for the last several decades," Obama told reporters in Tokyo.
"They are the most isolated country in the world," he continued. "They are subject to more international sanctions and international condemnation than any country in the world."