Further exercises are planned, U.S. officials say.

CNN: STORY HIGHLIGHTS

On Thursday, a spokesman for the South Korean Defence Ministry, Kim Min-seok, said Pyongyang had been observed carrying out "various drills involving the army, navy and air force, " and that further exercises were being prepared, the Yonhap news agency reports.

BBC: North Korea ramps up nuclear rhetoric as UN vote looms

But dialogue on Korean peninsula security, including the issue of military exercises, could help prevent further misunderstanding and miscalculation.

BBC: Viewpoint: What is driving North Korea's threats?