China confirmed Saturday that preparations were still ongoing for a World Health Organization mission to Wuhan to investigate the origins of Covid-19, following a rare rebuke from the UN body over a delay to the long-planned trip.

The comments came after the WHO chief said on Tuesday he was "very disappointed" that Beijing had yet to finalise permission even as the team of experts had begun travelling to China to explore the beginnings of the virus, which first emerged in late 2019 in the central city.

On Saturday, National Health Commission vice minister Zeng Yixin told reporters: "The specific time is being determined, and we are ready here."

"As long as these experts complete the procedures and confirm their schedule, we will go to Wuhan together to carry out investigations," he said.

Earlier in the week, Chinese authorities had refused to confirm details of the visit, a sign of the enduring sensitivity of the mission -- which has been beset with delays and politics.

The WHO previously said China had granted permission for a visit by a 10-person team.

"We are currently waiting for WHO's experts to arrive, and have arranged for relevant expert groups to receive them," Zeng added.

He expressed hope the WHO investigations could further understanding on the origins of the coronavirus.

Asked about the effectiveness of vaccination against a coronavirus strain found in Britain that appeared to be more infectious, Zeng added: "Our vaccine has the same neutralising ability against such a mutant strain."

"It seems that the mutation speed of the new coronavirus is still within an acceptable range, and the mutation speed is not particularly fast," he said.

Tight virus restrictions

Two Chinese cities south of Beijing further tightened virus restrictions Saturday as authorities race to stamp out a resurgence in infections.

China has largely brought its domestic outbreak under control after the coronavirus first emerged in Wuhan in late 2019 -- but a spike in Hebei province, bordering Beijing, has sparked fresh lockdowns ahead of the Lunar New Year next month.

Hebei has reported over 130 confirmed Covid-19 cases in the past week, with more than 200 more asymptomatic infections.

Most of the cases were in Shijiazhuang city, which along with its surrounding areas is home to 11 million people. Several other infections were reported in neighbouring Xingtai city, home to 7 million.

Shijiazhuang suspended subway operations from Saturday morning to aid "prevention and control of the outbreak", authorities said.

Major highways leading into the city, around 300 kilometres (200 miles) south of Beijing, have already been closed and inter-city passenger travel halted.

Overnight, Xingtai announced a week-long stay at home order to residents in order to curb its outbreak.

The curbs come ahead of the Lunar New Year, when hundreds of millions criss-cross China to visit family and friends, with National Health Commission vice minister Zeng Yixin warning Saturday the festival "will further boost the risk of transmission."

Authorities are racing to roll out vaccines, with more than 9 million doses given so far, Zeng added.

Health authorities recently gave conditional approval to a vaccine candidate by Chinese pharma giant Sinopharm, with emergency use jabs already administered in the later part of 2020.