(Adds new Omicron cases in third paragraph)

Dec 25 (Reuters) - Xian in northwest China reported an increase in daily COVID-19 infections for Friday, and local companies curtailed activity as the country's latest COVID-19 hot spot entered its third day of lockdown.

The city of 13 million detected 75 domestically transmitted cases with confirmed symptoms for Friday, its highest daily count of the year and reversing the previous day's decline, official data showed on Saturday.

In the southern city of Shenzhen, five passengers who arrived on Wednesday, on a flight from Los Angeles, tested positive for coronavirus, and three were confirmed to be Omicron infections on Dec. 24, China's state broadcaster CCTV reported.

Xian, which started a new round of mass testing on Saturday, has announced no infections caused by the Omicron variant. Nationwide, China has reported a handful of Omicron infections among international travellers and in southern China.

Xian's locally transmitted symptomatic cases, at 330 for the Dec. 9-24 period, are few compared with outbreaks in many other countries, but the city has imposed heavy-handed measures in line with Beijing's policy to contain local transmission as quickly as possible.

Residents are banned from leaving town without clearance from employers or local authorities and households can send only one person to shop for necessities every two days. Other family members may not leave home unless they have essential jobs or urgent matters approved by employers or communities.

"So far the turning point for the outbreak is yet to come, and stringent curbs are necessary," Zhang Boli, who helped to shape China's early COVID response and treatment, told state media.

Local officials have also faced punishment https://www.reuters.com/world/china/china-reports-87-new-covid-19-cases-dec-23-vs-100-day-earlier-2021-12-24 over the outbreak, and domestic flights scheduled to depart the city on Friday were cancelled.

XiAn ChenXi Aviation Technology, a supplier to the Chinese military, said late on Friday it had suspended production due to the outbreak, which it expects to hurt its 2021 operating income.

Western Securities said on Friday its Xian-based branches have stopped offering face-to-face services to clients.

An oncology hospital said on Saturday its outpatient department and emergency room had stopped receiving patients due to COVID-19, after several other hospitals made similar moves.

China's nationwide figures confirmed 87 local symptomatic cases for Friday, up from 55 a day earlier. There were 53 confirmed cases among international travellers. (Reporting by Shanghai and Beijing Newsroom; Editing by Edmund Klamann and William Mallard)