LONDON (Reuters) - Prime Minister David Cameron accused United States whistleblower Edward Snowden and unnamed newspapers of assisting Britain's enemies by helping them avoid surveillance by its intelligence services.

In his strongest remarks on the subject yet, Mr Cameron told a news conference in Brussels on Friday that the classified information which Snowden had leaked was going to make it harder for Britain and other countries to keep its citizens safe from people who wanted to "blow up" families.

"What Snowden is doing and, to an extent, what the newspapers are doing in helping him do what he is doing, is frankly signalling to people who mean to do us harm how to evade and avoid intelligence and surveillance and other techniques,"Mr Cameron told reporters.

"That is not going to make our world safer, it's going to make our world more dangerous. That is helping our enemies." Mr Cameron was talking after a European Council meeting in Brussels which had been overshadowed by allegations that the United States had tapped the mobile phone of German Chancellor Angela Merkel.

He declined to answer questions about Britain's possible involvement, saying only that its intelligence services routinely shared information with other European countries and were subject to proper oversight.

Mr Cameron has criticised Britain's Guardian newspaper, which has published many of Snowden's leaks in the past, but did not mention it by name on Friday. He said people needed to adopt a"cold-hearted" view of what the intelligence services did rather than what he referred to as a "lah-di-dah airy-fairy" view.

"There are lots of people in the world who want to do us harm, who want to blow up our families, who want to maim people in our countries. That is the fact." he said.