A Pakistani health official says a special anti-polio drive has been launched in Islamabad after traces of the polio virus were found in the city's sewage system.

The virus was detected weeks ago but so far no cases have been reported in Islamabad.

Dr Rana Safdar, who coordinated Pakistan's polio eradication centre, says the three-day vaccination campaign started on Monday. Safdar says police have taken steps to protect the polio workers.

Islamabad registered its last polio case 10 years ago.

The Taliban and other militants have in the past attacked vaccination centres and health workers because they perceive the vaccination drives as part of a Western conspiracy to sterilise Pakistani children or collect intelligence.

Pakistan is among a few countries in the world where polio is endemic.