* Hundreds have died since start of year
* Violence threatens to rip open fault-lines in Karachi
* Some Shi'ites are fighting back against majority Sunnis (Adds background)
By Imtiaz Shah
KARACHI, March 3 (Reuters) - A suspected suicide bomber attacked Shi'ite Muslims as they were leaving a mosque in Pakistan's commercial capital on Sunday, killing at least 25 people and wounding 50, police said.
"It was a powerful blast on a congested Shi'ite-dominated area," senior police official Shabir Ahmad Sheikh said of the attack, the latest signal that Sunni militant groups are stepping up pressure on the minority sect.
Military offensives and U.S. drone strikes against the Taliban in Pakistan in recent years have reduced the number of suicide attacks on government and military targets.
But Sunni groups, most prominently Lashkar-e-Jhangvi (LeJ), have escalated attacks against Shi'ites, who they believe are non-Muslims.
Bombings targeting Shi'ites have killed nearly 200 people in the city of Quetta alone since the start of the year.
In 2012, sectarian attacks and clashes climbed by 47 percent to 208, according to the Pak Institute For Peace Studies, a prominent Pakistani think tank.
While the Quetta carnage grabbed world attention, a Reuters inquiry into a lesser known spate of murders in Karachi suggests the violence is taking on a volatile new dimension as a small number of Shi'ites fight back.
Pakistan's Western allies have traditionally been fixated on the challenge posed to the nuclear-armed state by Taliban militants battling the army in the highlands on the Afghan frontier.
But a cycle of tit-for-tat killings on the streets of Karachi points to a new type of threat: a campaign by LeJ and allied Pakistani anti-Shi'ite groups to rip open sectarian fault-lines in the city of 18 million people.
Pakistani intelligence agents say the LeJ has become a major security threat in Pakistan, which is also struggling with a fragile economy, dilapidated infrastructure and widespread poverty.
Shi'ite frustrations are rising with each blast. Shi'ites fired weapons in the air on Sunday night in Karachi, a bustling metropolis plagued by ethnic and political violence and crime.
"The explosion was so massive it jolted the entire area," said witness Ali Reza. "Two flats and nearby shops caught fire after the explosion and balconies of various buildings collapsed."
Another witness, Muhammad Kazim, said women and children who were shopping nearby were wounded. (Additional reporting by Matthew Green; Writing by Michael Georgy; Editing by Robin Pomeroy and Stephen Powell)
Our Standards: The Thomson Reuters Trust Principles.
