LONDON (AlertNet) - Essential aid is struggling to reach around 90,000 people who have fled a Pakistani military offensive against Taliban positions in a region near Afghanistan, aid agencies said.

Without fresh water and medicine there is a strong chance that disease will spread amongst people who fled the region of South Waziristan -- focus of a bombardment by Pakistan's air force -- for two neighbouring areas.

"This is a very poor area and the people here had very little beforehand and are likely to have a lot of trouble now," said one aid worker based in Islamabad.

"The risk of things going bad are quite high."

A suicide bomber walked into the offices of the U.N.'s World Food Programme (WFP) in Islamabad last week and killed at five people.

The aid worker in Islamabad who spoke to AlertNet declined to be named to reduce the risk of being targeted by militants.

Pakistan's army is trying to repel increasingly brazen attacks by militants, often backed by the Taliban who are fighting a U.S.-led coalition in Afghanistan.

Earlier this year the army recaptured the Swat Valley close to Islamabad from militants in heavy fighting that forced about 2 million people to flee the area, one of the biggest emergency relief operations in the world this year.

But the aid worker in Islamabad told AlertNet that tight restrictions imposed by the Pakistani military and the dangers of working in the region made it harder to get aid to people who had fled the bombing this week in South Waziristan.

"There are some smaller, local aid agencies there but because of the limits the big agencies aren't able to work freely," he said.

He also estimated that there were up to another 350,000 civilians still inside South Waziristan.

Militants have launched a string of attacks across Pakistan over the last few days killing about 150 people.

As well as the WFP offices, targets included an army barracks, a police station and other civilian buildings. A suicide bomber killed 12 people in the city of Peshawar on Friday.

The U.N.'s refugee agency (UNHCR) said 80,000 people from South Waziristan had registered with authorities in the Tank and Dera Ismail Khan districts in North-West Frontier Province by the start of September.

"They are staying with host families or have rented rooms," UNHCR said in a statement. "If full-scale military operations are launched, the numbers of displaced people are likely to rise significantly."

More people have left South Wazirisitan over the last few weeks as rumours of an attack circulated, UNHCR said.

Pakistan's army has now said that a ground attack is imminent.

UNHCR said in September it handed out plastic sheets, sleeping mats and jerry cans to around 6,500 people and set up a stockpile of equipment nearby.

But again security concerns and limited access hinder its operations.

"The key challenge is security and humanitarian access to people," UNHCR said.

"The deteriorating security situation in Pakistan continues to hinder our humanitarian operations in the country, including our ability to assess needs, and provide and monitor relief assistance."

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