A mother has sparked debate after posting a picture on social media showing her wet nursing a friend's son.

Jessica Anne Colletti, from Pennsylvania, shared the photo - which shows her feeding both her own 16-month-old son, and her friend's 18-month-old son - on a pro-breastfeeding page.

The picture of Mrs Colletti, 26, and the two boys, who she dubs 'milk siblings', has divided users on Facebook, the Daily Mail reports.

'Happy World Breastfeeding Week!' Mrs Colletti wrote. 

'I watch her son while she works and have been feeding them both for a year! So much love between these milk siblings, it's a special bond between us all.'  

Mrs Colletti has been breastfeeding her friend's son since he was just five months, after he struggled with formula, and says her friend was very much 'on board' with the idea. 

'She was breastfeeding but didn't have time to pump - I was already breastfeeding my son and it just made sense to nourish him in the same way,' Mrs Colletti told Daily Mail Australia.

'My friend and her son live with me and my husband. She knows everything about me and trusts me with the care of her child,' Mrs Colletti said.

The post, which has since been liked more than 500 times, received a mix of positive and less supportive responses from users.  

'I wish that I could like this a million times!' One user wrote. 'Wet nurses saved many lives years ago,' another said. 

Others were less impressed.

'I'm sure she [the friend] is definitely NOT okay with it,' one person wrote. 'Sorry but that's strange,' another commented. 

Mrs Colletti says the criticism doesn't come as a surprise. 

'I'm glad to get so much support from breastfeeding advocates and am not surprised by critics - a lot of people are misinformed about breastfeeding relationships,' Mrs Colletti said.