* United Nations braces for more refugees

* Expanded army offensive expected

By Megan Rowling

LONDON, June 9 (Reuters) - The United Nations is preparing for some half a million people to flee Pakistan's northwest region of Waziristan if the government mounts a big operation against Taliban militants there, the U.N. aid chief said.

John Holmes, the U.N. under-secretary-general for humanitarian affairs, said on Tuesday that plans were being made to deal with the fallout from an anticipated expansion of the army's offensive to root out insurgents in the northern Swat valley, which has already displaced around two million people.

Holmes said the government appeared to be considering a similar strategy in the lawless tribal lands of Waziristan on the Afghan border -- known as a Taliban stronghold -- which would present a fresh challenge for stretched aid agencies.

"Half a million people or more might come out of Waziristan. They will not go to the same geographical area as the present (uprooted people), so it's a completely separate operation," he told Reuters in a telephone interview fron New York.

"That's why our contingency planning has been focusing on that and how we might manage, which is not going to be easy, and will require a further effort and more resources."

The exact number of civilians uprooted by fighting in the north remains fluid as the government is still registering people, and there have been estimates that 2.5 million people have been displaced. [nISL449110]

But most of those uprooted by the violence have been forced to flee their homes since the end of April, and are crammed into the houses of host families or camps.

SHORT OF FUNDS

Aid groups are struggling to cope with the surge in need for food and other relief. On May 22, the United Nations launched an appeal for $543 million to help those displaced by the conflict, but as of Tuesday, it had received only around a quarter.

Holmes said the slow response from international donors was putting the humanitarian response at risk.

"These are very costly operations and some of the agencies on the food and other sides are making clear they will not be able to continue with this for more than a few more weeks, unless some funding comes through," he said.

He added that he did not know why governments were reluctant to contribute, given the strong international interest in Pakistan's efforts to tackle hardline Islamic militancy, but warned the humanitarian crisis was unlikely to end quickly.

"Our assumption is that this is going to last for several months before everybody can go home at the very least, because conditions in the areas we have been able to get into, like Buner, suggest there's a lot of work to do to restore basic services and clear up," he said.

The United Nations has not yet been able to enter much of the conflict zone to check on how many people remain and on the conditions. Only the Red Cross has been given access to Mingora, the main town in Swat.

"We would like to get in there as soon as is practical, but for the moment ... the government does not want us to go in ... and it is not safe to go there, so that's not something that is going to happen in the very immediate future," Holmes said. (Reporting by Megan Rowling)

AlertNet news is provided by