"There continue to be reports of abductions, disappearances and acts of violence and intimidation in Sri Lanka," he said in a statement.
Without strong mechanisms for independent human rights reporting, it is difficult to assess the true scope of the problem, he said.
"We consistently call upon the government of Sri Lanka to take decisive action to tackle human rights abuses, including by taking action against those responsible for violations," Miliband said.
Recent weeks have seen a considerable number of high profile attacks on media freedom in Sri Lanka, he also said.
"We condemn such brazen attacks. Of particular concern was the murder on 8 January of the Chief Editor of the Sunday Leader newspaper, Lasantha Wickrematung