At least four people have been killed in renewed clashes in Karachi between Urdu-speakers and Pashtuns from northwest Pakistan.

Waseem Ahmed, the city police chief, said the four were killed in different incidents in the early hours of Tuesday but the city had been mostly calm since then.

"There has been no major incident since the morning," Ahmed told Reuters news agency.

The clashes broke out on Saturday, leaving at least 40 people killed and dozens more injured, according to police and hospitals.

Rivals fought gun battles and burned shops and cars in several parts of the city over the weekend.

Security forces have been given permission to use gunfire to try to dispease the fighting.

Tit-for-tat

Nawaz Sharif, the former prime minister, has raised the possibility of Indian instigation of the violence in Karachi as a response to last week's assault in Mumbai, which New Delhi has linked to Islamabad, although the government has not suggested any link.

Sharif said he was surprised by the timing of the Karachi violence.

"The killings in Karachi erupted suddenly after the Mumbai incident," Sharif told reporters. "I'm surprised how it erupted all of a sudden ... I think this needs to be looked in to thoroughly, which forces are involved in it."

All schools and colleges in Karachi were shut for a second day on Tuesday and public transport was thin.

Karachi has a long history of political, ethnic and religious violence. The latest clashes between ethnic-based factions have raised fears of a return to the chronic bloodshed that plagued the city in the 1990s.

Authorities said on Monday that the battles were between Muttahida Qaumi Movement (MQM) ruling coalition and the Pashtun Awami National Party (ANP), although both parties denied.