Singapore plans to buy four US-made F-35 jet fighters -- one of the most advanced warplanes ever built -- to update its ageing current fleet, the defence minister said Friday.

The city-state, which has one of Asia's best-equipped militaries, announced in January it had opted for the Lockheed Martin jets over rivals from Europe and China, and planned to first buy a small number before deciding on a full fleet.

Giving an update on the plans, Defence Minister Ng Eng Hen told parliament that Singapore "will request an initial acquisition of four F-35s, with the option of a subsequent eight if we decide to proceed".

The current price of F-35 ranges from $90 million to $115 million, he said, adding the cost had been steadily falling due to healthy orders from around the world.

Ng said the sale had to be approved by US Congress but had the backing of the US administration.

Singapore has for several years been assessing which fighter jet should replace its current fleet of F-16s, which are due to be taken out of service in around a decade.

The F-35 is a supersonic plane whose advanced stealth characteristic allow pilots to avoid detection by radars, according to Lockheed Martin.

Its advanced electronic warfare capabilities enable pilots to locate and track enemy forces, jam radars and disrupt attacks.

The Singapore military also has F-15 fighter jets, Apache helicopters and submarines in its arsenal.