U.S. President Barack Obama and his South Korean counterpart, Park Geun-hye, are warning North Korea against conducting another nuclear weapons test.
During a joint news conference in Seoul Friday, Mr. Obama said Pyongyang will get nothing except further isolation if it proceeds with a fourth nuclear test.
He added that America's commitment to South Korea will never waiver, and the U.S. and South Korea will stand "shoulder to shoulder" against North Korean provocation.
The South Korean leader said Mr. Obama's visit, the second leg of an Asian tour, sends a firm message that North Korea's provocations will not be tolerated.
Mr. Obama also used the news conference to wade into a historical dispute that has strained relations between South Korea and Japan, which he also visited this week. He said Japan's war time use of so-called comfort women was shocking, but urged Seoul and Tokyo to look forward in their relationship.
The U.S. leader also expressed condolences to the families of the hundreds of children lost in last week's ferry accident.
Before Mr. Obama wrapped up a two-day visit to Japan earlier Friday, the two nations released a joint statement on security and trade. The countries said they share strong concern about China's air defense zone in the East China Sea, but reaffirmed interest in building productive ties with Beijing.
Following a Thursday meeting in Tokyo with Japanese Prime Minister Shinzo Abe, Mr. Obama explicitly stated the disputed Senkaku Islands fall under the treaty obliging the United States to defend Japan if attacked. Beijing also claims the islands, known as Diaoyu in China.
After two days in Seoul, Mr. Obama will head to Malaysia, where he will hold talks and attend a state dinner with Prime Minister Najib Razak. He will be the first sitting U.S. president to visit Malaysia since Lyndon Johnson traveled there in 1966.
Mr. Obama's last stop will be the Philippines, which is also involved in a territorial standoff with China and has deepened its military cooperation with Washington as a result.
This is Mr. Obama's fifth visit to Asia since taking office in 2009. He has promised to make the Pacific region a greater economic, diplomatic and military priority for the United States.