Pakistani troops killed 20 rebels in a ground and air operation in the northwest, the military said, after a shaky peace deal was thrown into doubt when the Taliban suspended talks.

The deaths take the toll since Sunday when the military launched its operation in Lower Dir district to around 50, officials said.

"Twenty militants were killed today by the Frontier Corps troops in Maidan area of Lower Dir," the military media wing said in a statement.

Paramilitary troops and helicopter gunships bombed suspected bases in Lower Dir for a second day running, a military official earlier said.

Helicopter gunships targeted different militant hideouts, the official told AFP, on condition of anonymity.

"Eight security officials were also killed in two days of operation," another military official said, requesting anonymity.

US pressure

The Taliban earlier suspended talks with the government after Pakistan's military launched a fresh offensive against Taliban fighters in the northwest, under intense US pressure to stop the advance of the extremists in the region.

"Our council of leaders met on Sunday night and decided to suspend peace negotiations with the government in North West Frontier Province," said a spokesperson for Soofi Mohammad, the cleric who negotiated a peace deal between the two sides in February.

"We, however, still adhere to the February deal," Ameer Izzat Khan told AFP, referring to the highly controversial accord that put three million people under sharia law.

Tehreek-e-Taliban Pakistan (TTP), led by Baitullah Mehsud, a wanted tribal warlord called the operation "unwanted".

Demanding an immediate halt to the offensive, TTP spokesperson Maulvi Omar told AFP "the operation can harm the peace deal."

"If the operation is not stopped Taliban from other places will be compelled to extend support to Taliban in Dir," Omar said in a phone call from an unknown location.

Despite the deal, Taliban militants on Monday took control of a telephone exchange in Behrain town, 30 kilometres north of Swat valley, police officials said.

A remote-controlled bomb in the northwestern town of Lakki Marwat killed two people and wounded five police officials, police said.

Sharia courts

The Taliban promised to lay down their arms in exchange for sharia courts in a deal billed as the end of a nearly two-year brutal insurgency that ripped apart the once-peaceful Swat area.

But the agreement was followed by further encroachments in the region, and the government has been in talks with the militants to try to restore peace there.

President Asif Ali Zardari, who recently ratified the sharia accord, told AFP on Monday that troops had now ousted the Taliban from Lower Dir, 75 kilometres west of Swat.

But he said more police were needed on the ground to prevent the militants from regaining control there.

Zardari said the peace deal with the Taliban remained valid "until the NWFP (North West Frontier Province) government tells me otherwise."

"There will be a reassessment of the situation by the provincial government and if needed we'll come back to parliament and the parliament will decide," he said in an interview with foreign journalists.

Zardari said an offensive in Swat was a "possibility" but that the geography of the region "limits our capabilities."

"It is a very populated area... the casualties would have been heavy," he said.

Is Osama bin Laden dead?

Zardari said that Pakistani intelligence believes al-Qaeda leader Osama bin Laden is dead but acknowledged they had no evidence.

"The Americans tell me they don't know, and they are much more equipped than us to trace him. And our own intelligence services obviously think that he does not exist any more, that he is dead," Zardari told reporters.

"But there is no evidence, you cannot take that as a fact," he said. "We are between facts and fiction."

Zardari was responding to reports that Pakistani Taliban in the troubled Swat valley have said they would welcome Bin Laden if he wants to visit the former Pakistani hill resort now in the hands of Islamists.

The advance from Swat into the neighbouring district of Buner of hundreds of Taliban, whose brethren were ousted by the 2001 US-led invasion of Afghanistan, saw Washington brand the extremists "an existential threat" to Pakistan.

US Secretary of State Hillary Clinton warned that Pakistan was "basically abdicating to the Taliban and to the extremists." US Secretary of Defence Robert Gates called on the country's leaders to take action.

Clinton also voiced "concerns" for Pakistan's nuclear arsenal if the Taliban topples the government, according to the transcript of an interview with Fox News.

But Zardari told AFP on Monday Pakistan's nuclear installations were in "safe hands"

"All Pakistani nuclear installations are under extra security," he said.

The United States insists that Islamist extremists, historically supported by Pakistani intelligence, pose the greatest threat to the country and not arch rival India ? the traditional view of Pakistan's powerful military.

Across Pakistan, more than 1800 people have been killed in a wave of al-Qaeda and Taliban-linked extremist attacks since July 2007.