Drugmaker AstraZeneca said Saturday it was scouring its supply chain to find more doses of its Covid-19 vaccine for Southeast Asia, which is facing its most serious outbreak yet of the virus.

The statement from the Anglo-Swedish company -- which produces its vaccine in Thailand for use domestically and in neighbouring countries -- comes in the wake of a supply shortage which has sparked heavy criticism of Thai Premier Prayut Chan-O-Cha's administration.

Under the terms of AstraZeneca's agreement with Thai authorities, 180 million doses are due for production, one-third destined for the Thai market and the rest to be exported.

By the end of July AstraZeneca will have delivered 11.3 million doses for Thailand, according to James Teague, AstraZeneca's representative in the country.

Exports have still not begun, even as the region faces a particularly virulent wave of Covid-19.

"We are delivering in the fastest possible timeframe, however, given the gravity of the Delta variant, we are leaving no stone unturned to accelerate supply further still," Teague said in an "open letter to the people of Thailand".

"We are also scouring the 20+ supply chains in our worldwide manufacturing network to find additional vaccines for Southeast Asia, including Thailand."

But "a global supply crunch" for Covid-19 vaccines and a shortage of the materials required to make them made it difficult to provide a specific timeframe, he added.

The AstraZeneca contract was awarded last year to Siam Bioscience, a firm owned by King Maha Vajiralongkorn that has no track record of producing vaccines.

It set out to produce vaccines for nine countries including Indonesia, the Philippines, Malaysia, as well as Thailand.

Siam Bioscience has not commented on reports of insufficient production or late delivery.

But Thailand has been forced to change its vaccine strategy by importing millions of doses of Chinese vaccines.

Anger is mounting, with just five percent of Thailand's 70 million residents fully vaccinated.

Most Thais avoid talking about it openly because of harsh lese majeste laws that make it illegal to criticise the monarch. Those that do face between three and 15 years in prison.

Former opposition leader and billionaire Thanathorn Juangroongruangkit has already been accused under the law after he said the vaccination policy was too dependent on Siam Bioscience.

Thailand is among a host of Southeast Asian countries that kept infection numbers low during 2020 but now face record numbers of cases amid slow vaccination campaigns.