Some hospitals have postponed vaccination appointments citing lack of supply, the hospitals said, with one group saying the delay would affect nearly 40,000 people, just days ahead of the start of Thailand's main vaccination programme.

That drive depends on 61 million doses of AstraZeneca vaccine to be made locally by a firm owned by Thailand's king, which is making vaccines for use across Southeast Asia. The Thai government had used the Sinovac brand for early inoculations but this week said 11 million more of those had been ordered.

"Both AstraZeneca and Sinovac vaccines will be distributed to hospitals in all province throughout June ... there will be more than 6 million doses," said Kiattaphum Wongrachit, permanent secretary of the health ministry. Concern about availability has grown as Thailand suffers its deadliest outbreak so far, with only 2.7 million people having received a first dose of a coronavirus vaccine.

The government has been scrambling to source more and the ministry on Friday said it hoped to sign a contract next week for 20 million shots of the Pfizer/BioNTech vaccine. It received 1.8 million AstraZeneca shots on Friday, of 6 million due this month.

An additional 200,000 doses of that vaccine have been sourced from South Korea, a health ministry source told Reuters. Concerns had emerged about the production capacity of royal-owned Siam Bioscience, after the Philippines said its AstraZeneca order had been reduced and delayed.

James Teague, president of AstraZeneca Thailand, said on Friday that its Thai partner was on track in terms of quality control and regular doses would be delivered locally in June, then to Southeast Asian countries from July.