Sunni and Shiite assassins on motorcycles are fomenting sectarian and ethnic violence in Karachi, Pakistan’s commercial capital, with 21 drive-by murders in the last two weeks.

The government has responded by putting troops on the streets, banning all public meetings except funerals for a month and arresting 24 hard-line Sunni activists.

Still, fears are growing one of the world’s largest Muslim cities could be plunged into a round of religiously motivated revenge killings.

Even more ominous is the possibility al-Qaeda-linked extremists are using the murders to fan ethnic and sectarian hatreds in order to undermine Pakistan’s government and make Karachi, home to 18 million people, ungovernable.

The city, Pakistan’s financial capital and main port on the Arabian Sea, was wracked by ethnic and sectarian violence for much of the 1980s and 1990s as feuds between Sunnis and Shiites erupted in riots and mass murder.

Its tumultuous politics produced equally violent confrontations between the Muttahida Qaumi Movement (United National Movement, MQM), supported by the Urdu-speaking majority population, whose ancestors migrated from India at the time of partition in 1947, and the Awami National Party (ANP), supported by the ethnic Pashtun minority.

The latest round of violence began last month with targeted assassinations of MQM and ANP activists, and attacks on the Muhajir Qaumi Movement-Pakistan, a MQM splinter group, and the Islamistsupporting Jamaat-e-Islami.

June has seen a new round of mayhem in which most of the victims were Shiites.

Two of this week’s one-dozen murders involved drive-by shootings Thursday of a doctor as he left his clinic in Karachi’s Landhi suburb and a paramilitary Pakistan Ranger in the city’s central Garden district.

In both cases, hit squads on motorcycles drew up alongside the targets and opened fire with handguns and AK47s.

Five of the most recent victims were Shiite doctors, who appear to have been picked for because of their visibility in the community.

Karachi’s Shiite leaders blame the murders on the Islamist terrorist group Lashkar-e-Jhangvi, the armed wing of the now-banned Sipah-e-Sahaba (SSP, or Army of the Friends of the Prophet).

Lashkar-e-Jhangvi is suspected of being involved in the assassination of Benazir Bhutto, the former Pakistan prime minister, in Karachi in December 2007.

The SSP came into existence in Pakistan in the early 1980s as a reaction to the Iranian Revolution. It was allegedly created at the behest of Pakistan’s then-military dictator, Zia-ul-Haq, to counter an increasing Shiite militancy.

The SSP wants Pakistan to become a Sunni state and Shiites to be declared non-Muslims.

Over the years, its members have literally waged war on the Shiites. In December, they killed more than 40 of them in two bombings during a religious procession in Karachi.

SSP members also have close links with the Pakistani and Afghan Taliban, receiving arms training in Taliban camps, while offering Taliban members religious instruction in SSP-run madrassas in Pakistan.

The SSP was banned in Pakistan in 2002, around the time some members were accused of involvement in the beheading in Karachi of Daniel Pearl, a Wall Street Journal reporter. But the group has constantly re-emerged under different names.

Now that the United States has stepped up its fight against the Taliban in Afghanistan and pushed Pakistan’s military to root out Islamist fighters in the country’s northwestern tribal belt, the terrorists appear to be trying to destabilize Karachi.

“Anti-state elements are trying to disturb peace in the city and the country,” Rehman Malik, Pakistan’s Interior Minister, said yesterday. “The elements involved in the conspiracy first tried to ignite ethnic riots between the Pashtun and Urdu-speaking populations, but after their failure, they now want to start sectarian clashes.”

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