BEIJING, Sept. 5 (Reuters) - China National Biotec Group (CNBG) and Sinovac Biotech Ltd said on Saturday four more countries have agreed to run late-stage human tests of their coronavirus vaccine candidates, as China steps up its efforts in the global race.

Serbia and Pakistan are among the new countries agreeing to Phase 3 trials, as Chinese companies seek more data overseas for final approvals of potential vaccines. China's dwindling number of new infections make the country an unsatisfactory trial site.

The regulatory green light for mass use requires companies to prove the drugs are safe and effective enough with data collected from large-scale, Phase 3 trials for a potential vaccine against a virus that has infected more than 26 million people. COVID-19 has killed more than 870,000 people.

China has allowed two vaccine candidates from CNBG and one from Sinovac to be used in limited groups of people facing high infection risk, such as medical workers.

Serbia will trial both CNBG vaccines, and Pakistan will test the candidate developed by the firm's Beijing unit, as part of CNBG's Phase 3 human test scheme, its vice president Zhang Yuntao told Reuters.

The Phase 3 trials are expected to involve 50,000 people in about 10 countries, Zhang said. Trials have begun in United Arab Emirates, Bahrain, Peru, Morocco, Argentina and Jordan.

Foreign countries have expressed their intentions to order a combined 500 million doses of CNBG's vaccines, Zhang said.

Sinovac's vaccine candidate CoronaVac, being trialled in Brazil and Indonesia, has obtained approvals from two other countries for Phase 3 tests, said Helen Yang, senior director of global strategy and business development at Sinovac.

She declined to name the countries as the information is still confidential.