NEW YORK (AFP) - A 27-year-old US woman from New York state who killed her five-year-old son by poisoning him with salt was jailed for 20 years on Wednesday.

Lacey Spears documented her son Garnett's battle with a mysterious illness at length on social media. Prosecutors said she enjoyed the "attention and sympathy" she received from having a sick child.

After the 5-year-old died on Jan 23, 2014, Spears was charged with second-degree murder and first-degree manslaughter.

Judge Robert Neary said while Spears suffered from the mental illness known as Munchausen by proxy, she was guilty of a crime "unfathomable in its cruelty."

Spears, from Scottsville, Kentucky, was convicted of second degree murder last month.

During the trial, the court heard how the woman had given her son lethal doses of salt through a feeding tube because she craved the attention it gave her.

Judge Neary said the woman's son had suffered "five years of torment and pain."

"One does not have to be a psychiatrist to realise you suffer from Munchausen by proxy," Neary said as he resisted calls to give Spears a 25-year sentence sought by prosecutors.

Munchausen by proxy is a disorder which in some cases sees a caregiver fabricate or induce physical or health problems for those in their care.

District Attorney Janet DiFiore said Spears' son had suffered "repeated hospitalisations, unneeded surgical procedures and ultimately poisoning with salt - all at the hands of the one person who should have been his ultimate protector: his mother."

"Using the child's 'illnesses" to self-aggrandise herself, her actions directly lead to her son's tortured death," DiFiore said.