Kim Jong-nam wrote to Kim Jong-un in 2012 asking his half-brother and the recently anointed dictator of North Korea to spare his life and that of his family, the head of South Korea's National Intelligence Service revealed on Wednesday.

It comes as new pictures of a woman alleged to have been linked to the assassination team that killed the older brother of North Korean dictator Kim Jong-un in Kuala Lumpur have been released by Malaysian media.

Kim Jong-nam, 45, died on Monday after collapsing at Kuala Lumpur International Airport while waiting to board a flight back to Macau, where he was living in exile.

Malaysian authorities have detained a woman in connection with the investigation. The woman, in her 20s, was detained in the low-cost terminal of the Kuala Lumpur International Airport, Bernama news agency reported, citing a deputy inspector general of police.

Who are the suspects?

There are two female suspects and four male, police sources told the Telegraph. "One of the girls was told to hold a handkerchief on the face of the victim after he'd been sprayed by the other girl," an unnamed senior police officer said. "She held it there for 10 seconds. She said she thought spraying him had been a 'prank'."