Hong Kong has reported its first case of the H7N9 bird flu strain, in a possible sign the virus is spreading beyond mainland China since it first emerged there earlier this year.

The southern Chinese financial hub's health secretary said late on Monday that a 36-year-old Indonesian maid is in hospital in a critical condition.

He said the maid had a history of travelling across the border to the mainland city of Shenzhen to buy, slaughter and eat chickens.

H7N9 was first identified in April. It has sickened 139 people and killed 45 in China and Taiwan.

It appears to have stalled since Chinese authorities cracked down on live animal markets following the initial outbreak.

But scientists fear the virus will re-emerge in the northern winter, when influenza is most active.