The three new asymptomatic COVID-19 patients in Alashankou Port, Northwest China's Xinjiang Uygur autonomous region, hadn't traveled to places out of Xinjiang in the past two weeks and had no contact with confirmed, suspected or asymptomatic COVID-19 cases, an official said on Sunday.

The patients tested positive for the coronavirus on Sunday during regular nucleic acid testing in Alashankou Port, Bortala Mongol autonomous prefecture, and were diagnosed as asymptomatic. None of the individuals had visited medium- and high-risk areas for COVID-19 in the country. They have already been placed under quarantine and medical observation in a designated medical facility in the prefecture, said Song Tao, director of the prefecture's health commission, at a news conference on Sunday night.

Epidemiological surveys have been launched to trace the infected patients. Ninety-eight close contacts and 88 secondary close contacts were identified and put under quarantine and medical observation, Song added.

After the three asymptomatic cases were reported, the prefecture immediately launched nucleic acid tests to cover its residents.

The results of the 498,879 people who were tested have all turned out negative as of 8 am on Monday. The second round of citywide nucleic acid tests has already been launched in Alashankou, according to the prefecture's COVID-19 prevention and control headquarters.

"Citywide nucleic acid testing will be done once a day in Alashankou. Stricter disinfection efforts will also be carried out in quarantine centers, the asymptomatic patients' residential areas and public places they've passed through," said Zhang Mengfan, deputy secretary-general of the prefecture's government.

He added that Alashankou has also closed off key areas for COVID-19 prevention and control. Stricter nucleic acid testing and disinfecting measures have been conducted on imported goods that pass through Alashankou Port.