BANGKOK: AstraZeneca Plc has told Thailand it should be able to supply around 6 million doses of its COVID-19 vaccine per month, leaked correspondence showed, contradicting assertions by Thai officials that the government is owed 10 million.

Thailand's push for 10 million monthly doses comes as it considers imposing vaccine export curbs on Thai-manufactured vaccines to shore up domestic supplies, a move that could create problems for its neighbours, some of which are battling similar or more severe coronavirus crises.

But a June 25 letter by AstraZeneca to Thailand's health minister showed that the Anglo-Swedish drugmaker had offered to supply just 5-6 million doses a month to Thailand from a local plant, or one third of the amount produced by its partner Siam Bioscience, which is owned by Thailand's king.

"I hope you will be pleased that this is nearly twice the volume we discussed during our meeting," AstraZeneca's vice president of global corporate affairs, Sjoerd Hubben, said in the letter, referring to a September 2020 meeting with Thai officials.

At that meeting, Thai government officials estimated Thailand required around 3 million doses per month, Hubben wrote.

The correspondence was first reported by Isara news agency and later seen by Reuters.

The idea of Thailand limiting exports of locally produced vaccines first surfaced last week as the country suffers a crushing new wave of the coronavirus, with record numbers of cases several days in a row.

Thailand has inoculated just 5.2% of its population of more than 66 million so far.

Deputy Health Minister Sathit Pitutacha said last week that AstraZeneca had requested delays to the 61 million doses he said the company had promised to Thailand by the end of 2021. That commitment would have allowed Thailand to administer 10 million doses per month this year, he said.

"We want 10 million doses because the original plan was 10 million doses," Sathit said.

A source with knowledge of Thailand's AstraZeneca agreement said the 61 million doses, about a third of the local factory's agreed output, would be delivered to Thailand overall, but no monthly volume commitment had ever been agreed. Reuters has not viewed the full agreement.

The director-General of the Thai Disease Control Department, Opas Karnkawinpong, confirmed on Sunday that the June 25 letter was authentic. He told reporters the 3 million figure had been a rough estimate and Thailand had formally asked AstraZeneca in April 2021 to provide 10 million monthly doses.

He did not say whether AstraZeneca had agreed to that, and said it was understood that vaccine availability depended on output and volume might need to be agreed on a monthly basis.

AstraZeneca had no comment on Monday on the leaked letter.

The letter also detailed AstraZeneca's supply commitments of nearly 114 million doses elsewhere in Asia - a supply chain that has become uncertain due to potential export restrictions by the Thai government.

It planned to supply 50 million doses to Indonesia, 30 million to Vietnam, 16.5 million to the Philippines, 10 million to Taiwan and 6.4 million to Malaysia, the letter showed.

Opposition lawmaker Wiroj Lakkhanaadisorn said Thai authorities were prepared for neither a global supply shortage nor the severe impact of variants of concern.

"The government was over-confident about the situation when making these plans," he told Reuters on Monday. "They have been very slow and careless."