'Satisfactory conditions'

Holmes told Al Jazeera that he had visited Buner and the situation seemed "at least satisfactory" for the displaced to return.

"We have not been able to visit other areas so we can't say if conditions are right," he said.

In video

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 Pakistan's displaced queue for aid
 UN humanitarian chief visits Pakistan's Buner

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"The most important thing is that the process must be voluntary and the conditions do have to be right on the ground, in terms of security and the restoration of basic services like power and water, telephone, police force and so on," he told Al Jazeera.

"If they are right, of course we support return because no one wants people to stay where they are any longer than they need to."

Nearly 400,000 people are believed to have returned to Buner already.

But many residents of the Swat valley remain concerned about the situation there as sporadic clashes continue to break out despite the military controlling the towns and all major communication routes.

The volatile security situation was highlighted on Friday by fighting in the semi-autonomous region of Bajaur, also in the northwest of the country.

Bajaur clashes 

At least 20 people were killed in clashes, despite officials claiming to have pushed the fighters out of the region near the border with Afghanistan after an offensive in February.

The deadliest fighting took place in the village of Chinnar in the Charmang Valley, where an army convoy was ambushed, Jamil Khan, a local government official, told the Associated Press news agency.

Four pro-Taliban fighters and two soldiers were killed in the attack.

Two hours later, the military pounded the area with helicopter gunships and artillery, killing 10 fighters, Khan said.

At least four policemen were killed at a checkpoint near Khar, the main town in Bajaur, on Thursday. 

More than 30 suspected Taliban fighters were also killed in air raids in the tribal district of Orakzai, south of Pakistan's main northwestern city of Peshawar, military and government officials said.