A former North Korean diplomat who defected to the South has urged an ex-colleague missing in Italy to come and settle in Seoul.

Jo Song Gil, the North's acting ambassador to Rome, went into hiding with his wife in November and is seeking asylum, according to Seoul's intelligence authorities.

It would be the first high-profile defection of a North Korean diplomat since 2016 when the then deputy ambassador to London, Thae Yong Ho, switched sides to settle in Seoul.

Jo has not contacted Seoul's spy agency since he went into hiding, suggesting he was seeking asylum in a third country in the West, possibly the US, according to several media reports. 

But Thae, who said he once worked with Jo at Pyongyang's foreign ministry, wrote an open letter urging his ex-colleague to come to the South instead and work together to help the two Koreas reunify.

"I thought that I knew a lot about the South...through the Internet while serving overseas. But the South I actually experienced was far more democratic and economically prosperous than I imagined," Thae said in the letter posted on his blog.

"Sure, the South is not exactly a paradise. But it is a place where you and I could achieve the dream we all have," he said.

"Wouldn't it be our lifelong mission as diplomats to help the two Koreas reunify...and to pass the unified peninsula to our children?"

Since coming to Seoul, Thae has become a public speaker giving speeches about the reality of his impoverished but nuclear-armed former homeland and about ways to narrow down differences between the two neighbours that technically remain at war.

The 1950-53 Korean War that sealed the division of the two nations ended with an armistice instead of a peace treaty, with all civilian contacts across the border strictly banned.

Thae described it a "duty, not a choice" for diplomats like him and Jo to come to the South, stressing, "if you come to the South, other colleagues of ours may follow suit, which will help expedite the day when the two Koreas reunify."

"I will wait for you in Seoul!" he said.

About 30,000 North Koreans have fled repression and poverty and settled in the capitalist South, mostly by secretly crossing over the increasingly porous border with China.

Jo, who is in his 40s and known to be fluent in French and Italian as well as English, came to Rome in 2015.

He became temporary acting ambassador in October 2017, after Italy expelled the then ambassador Mun Jong Nam in protest at a nuclear test Pyongyang staged a month earlier in violation of UN resolutions.

Italy is an important diplomatic mission for Pyongyang, as it handles relations with the Rome-headquartered UN Food and Agriculture Organization and North Korea suffers from chronic food shortages.