BANGKOK — Health authorities in Thailand began their much-anticipated mass rollout of locally produced AstraZeneca vaccines on Monday, but it appeared that supplies were falling short of demand.

Hospitals have been posting notices for several days that some scheduled appointments would be delayed, adding to existing public skepticism about how many doses Siam Bioscience would be able to produce each month.

The government has said it will produce 6 million doses in June, then 10 million doses each month from July to November, and 5 million doses in December.

“The vaccines will be delivered as planned,” Prime Minister Prayuth Chan-ocha told reporters Monday morning as he visited a vaccination station at an indoor stadium in the capital, Bangkok.

Prayuth’s government has come under fierce criticism for failure to secure timely and sufficient vaccine supplies. It also face criticism for its reliance on Siam Bioscience, which is owned by the country’s king and had no previous experience in vaccine production.

Prayuth said the Health Ministry had confirmed that vaccinations could begin Monday in every province.