Pakistani militants, who have escalated attacks in recent weeks, killed at least 41 people in two separate incidents, officials said on Sunday.
The United States has long pressured nuclear-armed ally Pakistan to crack down harder on both homegrown militants groups such as the Taliban and others which are based on its soil and attack Western forces in Afghanistan.
In the north, 21 men working for a government-backed paramilitary force were executed overnight after they were kidnapped last week, a provincial official said.
Twenty Shi'ite pilgrims died and 24 were wounded, meanwhile, when a car bomb targeted their bus convoy as it headed toward the Iranian border in the southwest, a doctor said.
New York-based Human Rights Watch has noted more than 320 Shias killed this year in Pakistan and said attacks were on the rise. It said the government's failure to catch or prosecute attackers suggested it was "indifferent" to the killings.
Pakistan, seen as critical to U.S. efforts to stabilise the region before NATO forces withdraw from Afghanistan by the end of 2014, denies allegations that it supports militant groups like the Afghan Taliban and Haqqani network.
Afghan officials say Pakistan seems more genuine than ever about promoting peace in Afghanistan.
At home, it faces a variety of highly lethal militant groups that carry out suicide bombings, attack police and military facilities and launch sectarian attacks like the one on the bus in the southwest.
Witnesses said a blast targeted their three buses as they were overtaking a car about 60 km (35 miles) west of Quetta, capital of sparsely populated Baluchistan province.
"The bus next to us caught on fire immediately," said pilgrim Hussein Ali, 60. "We tried to save our companions, but were driven back by the intensity of the heat."
Twenty people had been killed and 24 wounded, said an official at Mastung district hospital.
Concern Over Extremist Sunni Groups
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 now, 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 a host of other issues, such as poverty, official corruption and chronic power cuts.
Pakistan's Taliban have carried out a series of recent 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 U.S.-led forces in Afghanistan.
The Taliban denies a rift exists among its leaders.
In the attack in the northwest, officials said they had found the bodies of 21 men kidnapped from their checkpoints outside the provincial capital of Peshawar on Thursday. The men were executed one by one.
"They were tied up and blindfolded," Naveed Anwar, a senior administration official, said by telephone.
"They were lined up and shot in the head," said Habibullah Arif, another local official, also by telephone.
One man was shot and seriously wounded but survived, the officials said. He was in critical condition and being treated at a local hospital. Another had escaped before the shootings.
Taliban spokesman Ihsanullah Ihsan claimed responsibility for the attacks.
"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 who are caught during fighting," he said.
The powerful military has clawed back territory from the Taliban, but the kidnap and executions underline the insurgents' ability to mount high-profile, deadly attacks in major cities.
This month, suicide bombers attacked Peshawar's airport on Dec. 15 and a bomb killed a senior Pashtun nationalist politician and eight other people at a rally on Dec. 22.