Pakistani militants, who have escalated attacks in recent weeks, killed at least 41 people in two separate incidents.

In the north, 21 men working for a government-backed paramilitary force were executed overnight after they were kidnapped last week.

Officials said they had found the bodies of the 21 men kidnapped from their checkpoints outside the provincial capital of Peshawar on Thursday.

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"They were tied up and blindfolded," a senior administration official said.

"They were lined up and shot in the head," said another local official.

One man was shot and seriously wounded but survived. He was in critical condition in hospital. Another escaped before the shootings.

A Taliban spokesman said: "We killed all the kidnapped men after a council of senior clerics gave a verdict for their execution. We didn't make any demand for their release because we don't spare any prisoners caught during fighting."

Elsewhere, 20 Shia pilgrims died and 24 were wounded when a car bomb targeted their bus convoy as it headed toward the Iranian border in the south-west, a doctor said.

Witnesses said a blast targeted their three buses as they were overtaking a car about 35 miles west of Quetta, capital of Baluchistan province.

"The bus next to us caught fire immediately," said pilgrim Hussein Ali. "We tried to save our companions, but were driven back by the intensity of the heat."

Twenty people were killed and 24 wounded, an official at Mastung district hospital confirmed.

The attacks challenge claims that military offensives have broken the back of hardline Islamist groups. The US has long pressured nuclear-armed ally Pakistan to crack down harder on both homegrown militant groups such as the Taliban and others that are based on its soil and attack Western forces in Afghanistan.

New York-based Human Rights Watch has noted more than 320 Shias had been killed this year in Pakistan and said attacks were on the rise. It said the government's failure to prosecute attackers suggested it was "indifferent" to the killings.

Pakistan, seen as critical to US efforts to stabilise the region before Nato forces withdraw from Afghanistan by the end of 2014, denies that it supports militant groups such as the Taliban.

Afghan officials say Pakistan seems genuine about promoting peace in Afghanistan. At home, it faces a variety of lethal militant groups that carry out suicide bombings, attack police and military facilities and launch sectarian attacks.

International attention has focused on al Qaeda and the Pakistani Taliban. But Pakistani intelligence officials say extremist Sunni groups, lead by Lashkar-e-Jhangvi (LeJ) are emerging as a major destabilising force in a campaign designed to topple the government.

Their strategy, the officials say, is to carry out attacks on Shi'ites to create the kind of sectarian tensions that pushed countries like Iraq to the brink of civil war.

As elections scheduled for next year approach, Pakistanis will be asking what sort of progress their leaders have made in the fight against militancy and other issues, such as poverty, corruption and chronic power cuts.

Pakistan's Taliban has carried out a series of bold attacks, as military officials point to what they say is a power struggle in the group's leadership revolving around whether it should ease attacks on the Pakistani state and join groups fighting US-led forces in Afghanistan.