The United States is “deeply concerned” about North Korea’s nuclear advances, a senior US official said after a US research institute predicted Pyongyang could possess as many as 100 nuclear weapons within five years.

South Korea’s president compared a deadly North Korean artillery barrage to the September 11 terrorist attacks on the United States, saying today that the South must strengthen defences and cannot let the North “covet even an inch of our territory”.

US President Barack Obama’s envoy began a rare trip to North Korea today as a senior US official warned of strong sanctions unless the communist nation rejoins international nuclear talks.

South Korean officials headed to North Korea today for their first Red Cross talks in nearly two years amid a push by the North to reach out to Seoul and Washington after months of provocations, including nuclear and missile tests.

A top White House adviser has personally delivered a pointed message to North Korean officials in New York, urging them to act on a nuclear disarmament pledge and telling them that US patience is limited, a US official said.