Singapore will deploy a huge tethered surveillance balloon to boost its maritime and air security, the defence ministry has announced.

The helium-filled "aerostat" will be equipped with radar equipment that can spot threats from as far as 200 kilometres (125 miles) away, the ministry said in a post on its website late Tuesday.

"It will be deployed sufficiently high enough so as to have a clear line of sight over Singapore's air and sea space," the ministry said.

"Existing systems are facing increasing constraints, mainly due to the construction of taller buildings which prevent the systems (from) establishing a clear line of sight," it added.

The Straits Times reported that the balloon will be able to scan up to Malacca in Malaysia for stray aircraft as well as detect small boats coming from Indonesia's Pekanbaru.

The US-made, 55-metre (180-foot) blimp will be operated by eight ground crew and can run at a height of up to 600 metres.

The defence ministry did not reveal the cost of the project.

Speaking on Tuesday, Defence Minister Ng Eng Hen said the balloon would save the government approximately Sg$29 million ($23.2 million) in operating costs every year by not having to rely on round-the-clock surveillance flights.

"For a small island state like Singapore, surveillance and early warning to give us sufficient reaction time to respond will always be a challenge but the aerostat will improve our surveillance capabilities significantly," Ng said.

Singapore has the largest defence budget in Southeast Asia, thanks to public funds generated by its substantial economic growth.

Surrounded by far larger neighbours Malaysia and Indonesia, it has pursued a robust defence strategy since being ejected from the Malaysian Federation in 1965.