South Korea said it was seeing indications that North Korea could be preparing to carry out a fourth nuclear weapons test, as Washington warned Pyonyang against doing so.

The Defense Ministry's spokesman, Kim Min-seok, on Tuesday said that Seoul had detected signs of activity at Punggye-ri, the site of North Korea's previous three nuclear tests near the country's northeastern coast.

"We are considering that North Korea could do a sudden nuclear test, and as the previous cases show, we are considering the possibility of a provocation disguised as a nuclear test," Mr. Kim said.

The statement came ahead of a visit to South Korea by U.S. President Barack Obama, who is scheduled to arrive in Seoul on Friday.

The Pentagon warned Pyonyang against conducting another nuclear test, following a test in February 2013, its first since 2009 and its first under the leadership of Kim Jong Un.

"We continue to call on North Korea to meet its international obligations and to stop provocative actions whatever they are," said Rear Adm. John Kirby, the Pentagon press secretary. "That only further destabilizes the situation on the peninsula."

The U.S. and South Korea recently completed an annual set of military maneuvers that involved thousands of service members from both countries.

The White House said Tuesday that the U.S. is watching North Korea "very closely" for any move toward an action such as a nuclear test.

"North Korea has a history of taking provocative actions, and we are always mindful of the possibility that such an action could be taken," White House Press Secretary Jay Carney said while traveling aboard Air Force 1 on Tuesday.

Subscribe to WSJ: http://online.wsj.com?mod=djnwires

South Korea said it was seeing indications that North Korea could be preparing to carry out a fourth nuclear weapons test, as Washington warned Pyongyang against doing so.

The Defense Ministry's spokesman, Kim Min-seok, on Tuesday said that Seoul had detected signs of activity at Punggye-ri, the site of North Korea's previous three nuclear tests near the country's northeastern coast.

"We are considering that North Korea could do a sudden nuclear test, and as the previous cases show, we are considering the possibility of a provocation disguised as a nuclear test," Mr. Kim said.

The statement came ahead of a visit to South Korea by U.S. President Barack Obama, who is scheduled to arrive in Seoul on Friday.

The Pentagon warned Pyonyang against conducting another nuclear test, following a test in February 2013, its first since 2009 and its first under the leadership of Kim Jong Un.

"We continue to call on North Korea to meet its international obligations and to stop provocative actions whatever they are," said Rear Adm. John Kirby, the Pentagon press secretary. "That only further destabilizes the situation on the peninsula."

The U.S. and South Korea recently completed an annual set of military maneuvers that involved thousands of service members from both countries.

The White House said Tuesday that the U.S. is watching North Korea "very closely" for any move toward an action such as a nuclear test.

"North Korea has a history of taking provocative actions, and we are always mindful of the possibility that such an action could be taken," White House Press Secretary Jay Carney said while traveling aboard Air Force 1 on Tuesday.

Subscribe to WSJ: http://online.wsj.com?mod=djnwires

South Korea said it was seeing indications that North Korea could be preparing to carry out a fourth nuclear weapons test, as Washington warned Pyongyang against doing so.

The Defense Ministry's spokesman, Kim Min-seok, on Tuesday said that Seoul had detected signs of activity at Punggye-ri, the site of North Korea's previous three nuclear tests near the country's northeastern coast.

"We are considering that North Korea could do a sudden nuclear test, and as the previous cases show, we are considering the possibility of a provocation disguised as a nuclear test," Mr. Kim said.

The statement came ahead of a visit to South Korea by U.S. President Barack Obama, who is scheduled to arrive in Seoul on Friday.

The Pentagon warned Pyongyang against conducting another nuclear test, following a test in February 2013, its first since 2009 and its first under the leadership of Kim Jong Un.

"We continue to call on North Korea to meet its international obligations and to stop provocative actions whatever they are," said Rear Adm. John Kirby, the Pentagon press secretary. "That only further destabilizes the situation on the peninsula."

The U.S. and South Korea recently completed an annual set of military maneuvers that involved thousands of service members from both countries.

The White House said Tuesday that the U.S. is watching North Korea "very closely" for any move toward an action such as a nuclear test.

"North Korea has a history of taking provocative actions, and we are always mindful of the possibility that such an action could be taken," White House Press Secretary Jay Carney said while traveling aboard Air Force One on Tuesday.

Subscribe to WSJ: http://online.wsj.com?mod=djnwires