The UN refugee agency says it has received reports of horrific abuses being committed in Mali and it anticipates up to 700,000 more people will be forced to flee their homes in the next few months.

UNHCR staff members have relayed horror stories of "witnessed executions and amputations", and said large amounts of money were being offered to civilians to fight against the French-backed Malian army and its supporters, according to the agency's spokeswoman, Melissa Fleming.

Reports of the use of child soldiers among the al-Qaida-linked Islamist groups and disappeared family members have also surfaced, she said.

"The refugees said they fled the recent military intervention, the lack of any means of subsistence and fear of the strict application of sharia law," Fleming said.

"They reported having witnessed executions and amputations, and mentioned that large amounts of money are being offered to civilians to fight against the Malian army and its supporters. According to the accounts from refugees there are children among the rebel fighters. People spoke of family members having disappeared."

The agency is planning for the additional displacement of up to 300,000 inside Mali and 407,000 more refugees flowing into neighbouring countries.

It is urgently reinforcing its teams across the region, she said, as thousands more refugees flee to Mauritania, Burkina Faso, Niger, Algeria, Guinea and Togo.

The agency said about 200,000 people were forced to leave their homes in northern Mali in 2012 and were on the move within the country, while 144,500 Malians fled to neighbouring countries.