(Adds political activists killed in Swat, explosives found in NWFP)

By Robert Birsel

ISLAMABAD, Jan 24 (Reuters) - Pakistani forces have cleared militant strongholds from three areas in the South Waziristan region on the Afghan border and 40 militants and eight soldiers have been killed in the fighting, the military said on Thursday.

The army is sending in reinforcements and tanks after a week of fighting with militants loyal to a Taliban commander the government said was behind the assassination of opposition leader Benazir Bhutto last month.

The clashes, in which nearly 150 militants and more than 20 government soldiers have been killed, has raised fresh concern about the nuclear-armed country in the run-up to a Feb. 18 election that could weaken President Pervez Musharraf's power.

Security forces had carried out operations in three parts of South Waziristan, the military said.

"These areas have been cleared of militant strongholds and hideouts," the military said in a statement.

"Forty miscreants have been killed in the last 24 hours and 30 miscreants have been apprehended while many injured," the military said. Eight soldiers were killed and 32 wounded.

The fighting is in strongholds of militant chief Baitullah Mehsud, who the United States has also said was behind Bhutto's assassination in a gun and bomb attack in Rawalpindi on Dec. 27.

Mehsud has been blamed for a string of attacks in a suicide bomb campaign that intensified after commandos stormed a radical mosque complex in Islamabad last July. On Wednesday last week, his men attacked and captured another fort in Waziristan.

Security forces have been battling al Qaeda-linked militants in South Waziristan for several years. The mountainous region, occupied by conservative, independent-minded Pashtun tribesmen, has never come under the full authority of any government.

Militants in South and North Waziristan also attack U.S.- and NATO-led foreign forces and Afghan government troops across the border in Afghanistan.

"APPEASING U.S., BRITAIN"

Separately, three activists from a Pashtun party opposed to militants were gunned down in the Swat valley in North West Frontier Province, where the military launched an offensive against armed followers of a radical cleric in November.

Police in the northwstern city of Peshawar said they found explosives near a bridge shortly before former prime minister and opposition leader Nawaz Sharif was due to address anti-Musharraf lawyers at a nearby court.

The United States and other allies hope next month's election will restore political stability after months of turmoil over Musaharraf's manoeuvres to stay on as president, and refocus efforts on tackling militancy.

Admiral William Fallon, the head of the U.S. military's Central Command, visited Pakistan for talks with army chief General Ashfaq Kayani on Tuesday.

Fallon told reporters in Florida last week that Pakistan was increasingly willing to fight Islamist militants and accept U.S. help, without saying what kind of support.

The United States has already announced plans to step up training of Pakistan's Frontier Corps, a paramilitary force recruited from tribal lands.

In North Waziristan, which is also on the Afghan border and where militants are also active, about 2,500 tribesmen protested against the military's attacks in South Waziristan.

"These operations are unjustified and are only meant to appease America and Britain," a Muslim cleric, Mehmood-ul-Hassan, told the protesters in Mir Ali town.

"The government should declare a ceasefire immediately. This can't be resolved through force but only through talks," he said. (Additional reporting by Hafiz Wazir, Haji Mujtaba, Junaid Khan and Faris Ali; Editing by Jeremy Laurence)

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