Beijing: North Korean leader Kim Jong-un has been detected by US sources walking in the port city of Wonsan, days after rumours spread that he was gravely ill after surgery.

South Korean newspapers have published accounts from sources who claim the leader was inspecting construction of the city's designated tourist area - a sprawling resort complex in the old trading port.

North Korean leader Kim Jong-un. Credit:AP

CNN reported on Tuesday the US was monitoring intelligence that Kim was "in grave danger" following surgery at his family's Hyangsan Clinic in North Pyongan province.

The Daily NK, a website run by North Korean defectors in Seoul, which maintains sources in the notoriously secretive regime, said excessive smoking, obesity, and overwork were the direct causes of Kim's urgent treatment and that he had been struggling with cardiovascular disease since August last year.