A spokeswoman for a British children's hospital says an operation is under way to separate conjoined twins.

The spokeswoman for Great Ormond Street Hospital says the decision to separate Faith and Hope Williams, who were born last week, was moved up after concerns for the girls developed.

The pair's mother is 18-year-old Laura Williams, who is Britain's youngest-ever mother to give birth to conjoined twins. The baby girls are joined from the breastbone to the top of the navel, but have separate hearts.

The spokeswoman says the final decision to separate the babies was made by Williams and her husband Aled.