Officials in South Korea are calling for caution amid reports that North Korean leader Kim Jong Un may be ill, emphasising that they have detected no unusual movements across the border in the north.

At a closed-door forum on Sunday, South Korea's Unification Minister Kim Yeon-chul, who oversees engagement with the North, said the government has the intelligence capabilities to say with confidence that there was nothing unusual happening.

Rumours and speculation over the North Korean leader's health began after he did not appear at a key state holiday on April 15. He has not been seen in public since.

South Korea media last week reported that Kim may have undergone cardiovascular surgery or was in isolation to avoid exposure to the new coronavirus.

However, they did carry reports that he had sent a message of gratitude to workers building a tourist resort in Wonsan, an area where some South Korean media reports have said Kim may be staying.

"Our government position is firm," Moon Chung-in, the top foreign policy adviser to South Korean President Moon Jae-in, said in comments to news outlets in the United States.

"Kim Jong Un is alive and well. He has been staying in the Wonsan area since April 13. No suspicious movements have so far been detected."

Personal train

Satellite images from last week showed a special train possibly belonging to Kim at Wonsan, lending weight to those reports, according to 38 North, a Washington-based North Korea monitoring project.

Though the group said it was probably the North Korean leader's personal train, Reuters news agency has not been able to confirm that independently, or whether he was in Wonsan.

A spokeswoman for the Unification Ministry said on Monday she had nothing to confirm when asked about reports that Kim was in Wonsan.

Last week China dispatched a team to North Korea including medical experts to advise on Kim Jong Un, according to three people familiar with the situation.

Reuters was unable to immediately determine what the trip by the Chinese team signalled in terms of Kim's health.

Experts have cautioned that Kim has disappeared from state media coverage before and that gathering accurate information in North Korea is notoriously difficult.

North Korea's state media last reported on Kim's whereabouts when he presided over a meeting on April 11.

Kim, believed to be 36, vanished from state media for more than a month in 2014 and North Korean state TV later showed him walking with a limp.