JOHANNESBURG -- The Head of the World Health Organization (WHO) urged people to limit use of mobile phones on Thursday, saying she suffered headaches whenever one was switched on nearby.

"I'd be cautious" about using mobile phones, WHO Director-General Gro Harlem Brundtland, 63, told Reuters on the sidelines of the Earth Summit in Johannesburg. "There's no reason to use them more than absolutely necessary or let children, for instance, talk on them for hours," she said. "We don't know that it's safe."

Surveys of whether microwave emissions from mobile phones can damage health have turned up no firm evidence of a link to tumors or cancer, she said. "But we are making bigger surveys that are not yet completed."

Brundtland, a former Norwegian prime minister who also wrote a landmark 1987 report on the environment, said that she suffered headaches whenever a mobile phone was turned on within a radius of about 2-3 meters (7-10 ft) from her.