Thailand's navy today defended its plan to buy three submarines from China amid growing public criticism and questions over the need for the costly vessels.

The military government approved last week the purchase of the first submarine from China, at a cost of 13.5 billion baht (US$390 million).

Navy chief of staff Admiral Leuchai Ruddis said the payment will be made in installments over seven years and will not leave other parts of the government's budget short.

"There is an uncertain security situation in our region of the world and the need for force can happen at any time," Leuchai told a news conference at the navy's headquarters in Sattahip, south-east of Bangkok.

Leuchai said Thailand had to defend its maritime borders and he shrugged off public concern.

"It is necessary to move forward with the purchase," he said.

Thailand is not involved in disputes with China and other South-East Asian countries over the South China Sea, but it does have a dispute with Cambodia over their maritime border in a gas-rich part of the Gulf of Thailand.

Thailand also has maritime borders in the Indian ocean.

Politicians from both main political parties have criticised the purchase of the S26T submarines, accusing the military government of mismanaging priorities at a time of economic downturn.

Thailand's defence budget this year is more than 210 billion baht (US$6.11 billion), about nine percent more than in 2014, the year the military took power.

China was the first major power to acknowledge Thailand's ruling junta following the 2014 coup.

The United States responded to the overthrow of an elected government by freezing US$4.7 million of security-related aid and cancelling some security agreements.

- Reuters