By Kamran Haider

ISLAMABAD, May 25 (Reuters) - Pakistani soldiers were moving from house to house on Monday as they battled militants in the main town in the Swat valley and were expected to take at least a week to secure it, the military said.

The offensive in the Taliban bastion of Swat, about 120 km (80 miles) northwest of Islamabad, is the military's most concerted effort to roll back a spreading Taliban insurgency that has thrown the nuclear-armed country's future into question.

"Look at the size of the city, the number of militants present there, the number of houses to be searched," said military spokesman Major-General Athar Abbas.

"It'll take at least a week to clear it," he said.

Concern was growing for the thousands of civilians believed left in the town and a senior U.N. official said a request for a "humanitarian pause" was being considered.

The army launched the offensive this month after the militants, emboldened by a controversial peace deal, pushed out of the former tourist valley to conquer neighbouring districts, including one just 100 km (60 miles) northwest of Islamabad.

The United States, which needs Pakistani action against militants in its northwest to defeat al Qaeda and disrupt support for the Taliban in Afghanistan, decried the pact as tantamount to "abdicating" to the militants.

The United States, which is pouring thousands of extra troops into Afghanistan to try to reverse Taliban gains, welcomed the subsequent resolve which the Pakistani army has demonstrated in its bid to clear Swat.

While Swat is not on the Afghan border, there have been fears it could turn into a fortress for al Qaeda as well as Taliban fighting in both Pakistan and Afghanistan.

The fighting sparked an exodus from valley which has posed a major problem for an economy being kept afloat by a $7.6 billion International Monetary Fund loan.

According to provincial government figures, 2.38 million people have been displaced while about 555,000 had fled earlier fighting in the region, a U.N. refugee agency spokeswoman said.

That figure had to be verified and could come down if instances of double counting were found, she said.

The United Nations has appealed for $543 million in aid.

"ON THE RUN"

About 15,000 soldiers are fighting 4,000 to 5,000 militants in Swat, the army says. The government says about 1,100 militants and about 60 soldiers have been killed although there has been no independent confirmation of those estimates.

Abbas said soldiers had captured several important intersections in Mingora and the militants were fleeing in some areas but battling hard in others.

"Everything is going according to the plan, the strategy. They're on the run though they're also putting up resistance at different places," he said.

Among the latest incidents, a bomb exploded when militants were planting it northwest of Mingora, killing six of them, while soldiers foiled a suicide attack by firing at a car loaded with explosives approaching a checkpost, military officials said.

The military said soldiers had captured Maalam Jabba, a militant training centre which is also home to Pakistan's only ski resort, while heavy fire had taken place in the militants' Peochar valley stronghold.

While many civilians have fled, many are still left including an estimated 10,000 to 20,000 in Mingora and about 200,000 in the north of the valley, where there has been less fighting.

Shocked by the brazen Taliban advances, the offensive has public support but that could wither if many civilians die.

A top U.N. official said the United Nations was considering asking the military to temporarily halt its offensive to provide aid to those trapped.

"A humanitarian pause is a subject of discussion and with the very good liaison we have with the armed forces, it is obviously something that we would not shy away from asking for," Manuel Bessler, head of the U.N. Office for the Coordination of Humanitarian Affairs in Pakistan, told AlertNet. (Writing by Robert Birsel; Editing by Jeremy Laurence)

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