It is two months since gunmen rampaged through the labyrinthine Shershah car-parts market in Karachi, leaving 13 shopkeepers dead. But business is still bad. There is fear in the faces and voices of traders. Violence between ethnic groups has been escalating in Pakistan's largest city and politicians are accused of abetting it.

The slaughtered traders were Mohajirs, a group whose ancestors migrated from northern India at the time of Pakistan's partition from India in 1947 (mohajir is an Arabic word meaning migrant). The killers are believed to have been Baluchis, whose ethnic homeland is the province of Baluchistan in western Pakistan.

"We are not linked to any political party. We were just doing our business. What was our fault?" says one trader. He describes how the gunmen pulled up the steel shutters of his little shop, shot dead his two sons, aged 24 and 26, and injured a brother who died a few weeks later.

Over the past year, territorial struggles between criminal gangs of Mohajirs, Baluchis and Pashtuns from Pakistan's north-west have fuelled Karachi's biggest outbreak of communal violence since the mid-1990s. A Western diplomat in Pakistan ventures that Karachi could even descend into "civil war".

Containing the violence is proving hard because of the ethnic affinities of Karachi's main political groups. Many in the city suspect that a lot of the gangsters have ties with politicians, for whom expanded territorial control can translate into more votes. Party workers sometimes forcibly take identity cards from voters in their areas and use them to cast fraudulent ballots.

The dominant political party in Karachi, the Muttahida Quami Movement (MQM), sides with the Mohajirs, the city's largest ethnic group. The next biggest group, the Pashtuns, tends to back the Awami National Party. The Pakistan People's Party (PPP), which has led a coalition government in Islamabad since 2008, draws support from Baluchis.

Zulfiqar Mirza, the PPP home minister of Sindh, of which Karachi is the provincial capital, admitted on December 13 that all political parties were implicated in the killings in Karachi. He said that 60 suspected assassins were in custody, 26 of whom belonged to the MQM.

The MQM has some reason to worry about its future political strength. Pashtun ranks have been swelled by tens of thousands of people fleeing the north-west, where troops are battling the Taliban. "There's a plan to snatch Karachi from the MQM," says Haider Abbas Rizvi, an MQM member of the national parliament from Karachi. Rizvi denies that his party has any gang ties.

In Islamabad, the MQM is in a coalition that includes the other two parties. So the feuding in Karachi could destabilise the national government, which looks wobbly. A small Islamic party, the Jamiat e Ulema-e-Islam (JUI), said on December 14 that it was withdrawing from the coalition because of the sacking of a JUI minister.

Distraction

The violence is distracting the authorities in Karachi from tackling other threats. The city is an important hideout and fund-raising centre for the Taliban, Al Qaida and other militants. Last month, suspected militants detonated a car bomb outside Karachi's counter-terrorist police headquarters, killing at least 18 people.

But the death toll from religious extremism, which plagues the rest of Pakistan, is far exceeded in Karachi by that from ethnic violence. The assassination in August of an MQM member of the provincial parliament, Raza Haider, led to three days of bloodshed in which at least 80 people were killed, most of them Pashtuns.

The Citizens-Police Liaison Committee, a watchdog, says more than 1,100 have died in such violence this year, mainly in the city's poorer areas. Karachi is being rent asunder by the politicians' mobsters.