"We are inside the range rings of more and more countries that have missiles of various kinds, so we have to take that into account when we design our future air defence systems -- our radars, our missiles, our air defence fighters," Teo Chee Hean said in an interview with the Straits Times.

"We don't expect that they will be a direct threat to us. But the fact of the matter is that we are now within their range rings when, prior to that, we were not," he said without naming any countries.

"So this is something which, if we're prudent, we will take into account."

Singapore, a tiny but affluent island-nation, has one of Asia's most modern military arsenals and spends around six percent of gross domestic product annually on defence.

Last December, Singapore signed a contract to buy US-made Boeing F-15 fighter jets, in a further upgrade for its inventory which also includes Apache attack helicopters, stealth frigates and submarines.

Teo said Singapore was on track to transform its military into a "third generation" armed force that will make more use of technology in the battlefield. Units will also be smaller and more autonomous under the set-up.

"Over the next five to seven years, I think you'll see the major impact of this transformation coming... but this is an ongoing process and I would expect that it's something that will take us much of a decade to do so," he said.

The armed forces must adapt to changes in the nature of warfare and the emergence of new threats such as terrorism, the defence chief added.

Singapore has a battalion specialising in the protection of key installations, and another which focuses on developing doctrine and equipment required for urban warfare.

"At a higher level, what we see as changes to the security landscape are things like the proliferation of weapons of mass destruction, the proliferation of missiles with longer ranges -- ballistic missiles, cruise missiles," Teo said.

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