He was speaking at the opening ceremony of a meeting in Singapore of Asean telecommunications and information technology ministers.
"We must not condone such malicious and harmful behaviour," the Straits Times quoted him as saying.
"We must strengthen our defences and cooperate to deal with these common threats." Lee was speaking after a spate of recent attacks targeting Singapore, Thailand and the Philippines.
The official websites of Lee and Singapore's president were hacked last week, apparently by members of the shadowy activist group Anonymous.//DPA