The World Health Organization says what it calls a “dangerous strain” of polio has spread from Pakistan to China.

The U.N. agency says it has confirmed a genetic link between wild polio virus in China and a strain circulating in Pakistan.

It says there have been seven confirmed cases over the past two months in China's far western Xinjiang province, which borders Pakistan.

The WHO is warning that this strain of polio spreads more easily than others. It has urged countries to strengthen disease surveillance systems and says travellers to Pakistan should be vaccinated against polio.