Polio has been found in China for the first time since 1999 after spreading from Pakistan, the World Health Organization (WHO) has confirmed.
It said a strain of polio (WPV1) found in China was genetically linked with the type now circulating in Pakistan.
At least seven cases have now been confirmed in China's western Xinjiang province, which borders Pakistan.
The WHO warned there was a high risk of the crippling virus spreading further during Muslim pilgrimages to Mecca.
Polio (also called poliomyelitis) is highly infectious and affects the nervous system, sometimes resulting in paralysis.
It is transmitted through contaminated food, drinking water and faeces.
'Right things done'
On Tuesday, the WHO said the polio cases in Xinjiang had been detected in the past two months.
The Chinese authorities are now investigating the cases, and a mass vaccination campaign has been launched in the region.
"So far all the right things are being done," WHO spokesman Oliver Rosenbauer told Reuters news agency.
Polio was last brought into China from India in 1999. China's last indigenous case was in 1994.
Pakistan is one of a handful of countries where polio remains endemic.
WHO officials had been warning for some time that the virus was spreading within the country to previously uninfected areas.
The UN's children fund, Unicef, has said that eradicating polio from Pakistan depends on delivering oral vaccines to each and every child, including the most vulnerable and the hardest to reach.
Polio was virtually eliminated from the Western hemisphere in the 20th Century.