Specialist anti-North Korean websites and organisations run by defectors in South Korea said they were the victims of a co-ordinated cyber attack.
Free North Korea Radio, Daily NK and North Korea Intellectuals Solidarity, all operated by defectors from North Korea, said their servers crashed simultaneously on Tuesday afternoon, paralysing their websites.
The apparent intrusion followed last week's large-scale cyber attacks on South Korean broadcasters and banks, which was widely suspected to be the work of organised North Korean hackers.
Lee Sang-Young, an official with Daily NK, said the internet news provider's server was down for 50 minutes. "We lost all the stories filed this morning," he said.
Last week, the website of the Washington-based Committee for Human Rights in North Korea was disabled for several hours.