The campaign, launched nationwide Tuesday in cooperation with the World Health Organization, has been extended to five days, they said.

Vaccination teams focused on border regions near Afghanistan, where ease of movement between the two countries is seen as especially problematic in curtailing the spread of the crippling disease.

"The campaign went very well in the border regions, including Bajaur, South Waziristan, Mohmand and Khyber," health ministry official Mazhar Nisar Sheikh told AFP.

Earlier this week, armed men briefly abducted a group of health workers in the Bajaur area of remote North Western Frontier Province as they attempted to administer polio drops to local children.

Eleven health workers were held for four hours as their captors smashed vaccination kits after hearing rumours that the drive was a "US plot" to sterilise Muslim children.

Sheikh said no more incidents had been reported.

But he said vaccination teams had not been able to reach all areas of volatile North Waziristan because of the poor security situation in the area.

North and South Waziristan are two of seven conservative tribal regions near Pakistan's border where fiercely-independent tribes have been accused of giving shelter to Taliban and Al-Qaeda militants blamed for plotting bombings and other attacks in Pakistan, Afghanistan and around the world.

A Pakistani health official in charge of a polio inoculation campaign was killed in Bajaur area in a bomb blast in February.

Health officials had been trying to dispel rumours -- sometimes spread by radio stations or from the loudspeakers of mosques -- that the polio campaign was a Western conspiracy to reduce Muslim populations.

"We are confident of achieving our target to vaccinate 32 million children at the end of the five-day campaign," Sheikh said.

This is the second polio vaccination campaign in the last seven months. The government plans to launch two more nationwide drives in October and December.

Pakistan stands with neighbouring Afghanistan, India and Nigeria as one of four countries where polio is endemic.

The number of annual infections in Pakistan was 20,000 to 30,000 in the early 1990s before the inoculation drive, known as the Expanded Programme on Immunization, was launched in 1994.