Former US Congressman Anthony Weiner has been jailed for 21 months for sexting a 15-year-old girl on social media.

Mr Weiner, 53, cried in a New York court as he was handed the sentence, which includes an additional three years of supervised release, CNN reported.

Mr Weiner, a one-time rising star in the Democrats and estranged husband of former top Hillary Clinton aide Huma Abedin, pleaded guilty in May to one charge of transferring obscene material to a minor.

The girl, 15, first contacted Mr Weiner on Twitter in January 2016. They continued speaking until March that year on Facebook Messenger, Skype, Kik, Confide and Snapchat.

In a sentencing memorandum, prosecutors said Mr Weiner asked the girl to “engage in sexually explicit conduct via Skype and Snapchat, where her body was on display, and where she was asked to sexually perform for him”.

He was fully aware of her age, prosecutors said.

Southern District of New York Judge Denise Cote added Mr Weiner was also in contact with 19 other women. The defence reportedly argued this showed he did not have an interest in teenagers.

Mr Weiner will be registered as a sex offender and participate in sexual-offender treatment to be approved by the probation department.

The judge said Mr Weiner’s habit of exchanging sexually explicit messages and pictures with young women shows a “very strong compulsion” — so strong that “despite two very public disclosures and the destruction of his career on two occasions, he continued with the activity”.

Anthony Weiner with his wife Huma Abedin in 2013. Photo: Getty

Calling himself “a very sick man for a very long time”, Mr Weiner tearfully apologised to the girl and sought to assure the judge he had finally learned his lesson. He has also been undergoing therapy.

“I stand before you because I victimised a young person who deserved better,” he said.

“Your honour, I’m not asking that you trust that my recovery is real. I ask you for the opportunity to prove that it is real.”

Wearing a wedding band, he also spoke of his devotion to the five-year-old son he has with his wife, Ms Abedin, formerly Mrs Clinton’s closest aide.

The couple is going through a divorce.

Judge Cote said his sentence carried more weight because of his notoriety.

Anthony Weiner’s political scandals

Mr Weiner resigned from Congress in June 2011 after posting a suggestive picture on Twitter.

In 2013, he announced his candidacy for New York mayor. Screenshots emerged of sexual conversations he had with a woman the year prior emerged two months later.

Mr Weiner was embroiled in the email scandal of the 2016 Presidential Election, involving his now estranged wife Ms Abedin. It was revealed Mrs Clinton had sent emails containing classified information to Ms Abedin, who forwarded them to a computer used by Mr Weiner.

Former FBI Director James Comey decided to alert Congress in the lead-up to the election about his investigation into the email scandal.

The FBI cleared Mrs Clinton of any wrongdoing two days before the election.

– with AP